GTK: Reenable deprecation warnings

At this point we got a fully (?) functional gtk3 port, but it uses a ton
of deprecated functions that will be removed in gtk4.  Better enable the
warnings so that we know what to fix before then.

desmume/src/frontend/posix/gtk/meson.build

@@ -28,7 +28,6 @@ includes = [
 
 executable('desmume',
   desmume_src,
-  cpp_args: ['-DGDK_DISABLE_DEPRECATION_WARNINGS'],
   dependencies: gtk_dependencies,
   include_directories: includes,
   link_with: libdesmume,