Fix return type errors.

[hillwoodroc]

Fix build on openSUSE.

[ 79s] gpm-manager.c: In function 'gpm_manager_engine_close_notify_cb':
[ 79s] /usr/include/glib-2.0/glib/gmacros.h:796:15: error: 'return' with a value, in function returning void [-Werror=return-type]
[ 79s] 796 | #define FALSE (0)
[ 79s] | ^
[ 79s] gpm-manager.c:1241:10: note: in expansion of macro 'FALSE'
[ 79s] 1241 | return FALSE;
[ 79s] | ^~~~~
[ 79s] gpm-manager.c:1238:1: note: declared here
[ 79s] 1238 | gpm_manager_engine_close_notify_cb (GpmEngine *engine, GpmManager *manager)
[ 79s] | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[ 79s] gpm-manager.c: In function 'gpm_manager_engine_charge_critical_notify_cb':
[ 79s] gpm-manager.c:1641:25: warning: unused variable 'policy' [-Wunused-variable]
[ 80s] 1641 | GpmActionPolicy policy;
[ 80s] | ^~~~~~
[ 80s] gpm-manager.c: In function 'gpm_manager_init':
[ 80s] gpm-manager.c:2046:10: warning: unused variable 'error' [-Wunused-variable]
[ 80s] 2046 | GError *error = NULL;
[ 80s] | ^~~~~
[ 80s] cc1: some warnings being treated as errors