Gate the Spectranet page commands behind BUILD_SPECTRANET

[morozov]

#70 added the gdbserver remote commands spectranet-set-page and spectranet-set-pagein to gdbserver_remote_commands.c, which compiles regardless of BUILD_SPECTRANET. spectranet-set-page calls spectranet_map_page(), whose definition sits inside #ifdef BUILD_SPECTRANET in spectranet.c with no counterpart in the !BUILD_SPECTRANET stub block — so any Spectranet-off build (e.g. --without-pthread) fails to link:

gdbserver_remote_commands.c: undefined reference to `spectranet_map_page'

Fix: guard the two commands and their remote_commands[] entries with #ifdef BUILD_SPECTRANET, merged into the adjacent xfs-debug guard, so they are absent when Spectranet isn't built.

[desertkun]

Thanks, sorry for the mess