Add `cppcheck` to CI

[m-roberts]

Note that this will require some extra includes:

src/oshelper.cpp:4:0: information: Include file: "crypt.h" not found.
src/oshelper.cpp:6:0: information: Include file: "shadow.h" not found.

There are also some missing system includes, although cppcheck says that it does not need standard library headers to get proper results:

src/fileio.h:4:0: information: Include file: <QObject> not found.
src/oshelper.h:4:0: information: Include file: <QObject> not found.
src/config.h:7:0: information: Include file: <QJsonObject> not found.
src/config.h:8:0: information: Include file: <QObject> not found.
src/ptlogger.h:4:0: information: Include file: <syslog.h> not found.
src/ptlogger.h:5:0: information: Include file: <QCoreApplication> not found.
src/ptlogger.h:6:0: information: Include file: <QDateTime> not found.
src/ptlogger.h:7:0: information: Include file: <QDebug> not found.
src/ptlogger.h:8:0: information: Include file: <QJsonDocument> not found.
src/ptlogger.h:9:0: information: Include file: <QJsonParseError> not found.
src/ptlogger.h:10:0: information: Include file: <QProcess> not found.
src/ptlogger.h:11:0: information: Include file: <QThread> not found.
src/config.cpp:5:0: information: Include file: <QDebug> not found.
src/config.cpp:6:0: information: Include file: <QJsonArray> not found.
src/config.cpp:7:0: information: Include file: <QJsonDocument> not found.
src/config.cpp:8:0: information: Include file: <QJsonObject> not found.
src/console_log_handler.cpp:1:0: information: Include file: <QWebEnginePage> not found.
src/console_log_handler.h:4:0: information: Include file: <QObject> not found.
src/console_log_handler.h:5:0: information: Include file: <QDebug> not found.
src/fileio.cpp:2:0: information: Include file: <QDebug> not found.
src/fileio.cpp:3:0: information: Include file: <QDir> not found.
src/fileio.cpp:4:0: information: Include file: <QFile> not found.
src/fileio.cpp:5:0: information: Include file: <QTextStream> not found.
src/main.cpp:1:0: information: Include file: <QCommandLineOption> not found.
src/main.cpp:2:0: information: Include file: <QCommandLineParser> not found.
src/main.cpp:3:0: information: Include file: <QGuiApplication> not found.
src/main.cpp:4:0: information: Include file: <QQmlApplicationEngine> not found.
src/main.cpp:5:0: information: Include file: <QScreen> not found.
src/main.cpp:6:0: information: Include file: <QThread> not found.
src/main.cpp:7:0: information: Include file: <QtWebEngine> not found.
src/main.cpp:8:0: information: Include file: <QUrl> not found.
src/unix_signal_manager.h:4:0: information: Include file: <QCoreApplication> not found.
src/unix_signal_manager.h:6:0: information: Include file: <signal.h> not found.
src/unix_signal_manager.h:7:0: information: Include file: <unistd.h> not found.
src/oshelper.cpp:9:0: information: Include file: <QCoreApplication> not found.
src/oshelper.cpp:10:0: information: Include file: <QDebug> not found.
src/oshelper.cpp:11:0: information: Include file: <QDir> not found.
src/oshelper.cpp:12:0: information: Include file: <QProcess> not found.
src/oshelper.cpp:13:0: information: Include file: <unistd.h> not found.