This among others makes sure that the '-I'/'-isystem' compiler
flags for the gtk3 includes end up in the 'INCLUDE' section
in 'workdir/GbuildToJson/Library/libvclplug_gtk3lo.so' when
that file is generated by 'gbuildtojson'.

That file is used by 'gbuild-to-ide' to generate IDE-specific
config files.
Having the gtk3 headers in the 'INCLUDE' section makes sure that
the includes are properly taken into account when generating
the IDE-specific files, and the included files can thus
actually be found.

Also, there's no need to set '$$(INCLUDE)' via
'gb_Library_add_cxxflags', since the flags are already set via
'gb_Library_set_include', so drop the 'gb_Library_add_cxxflags'
part altogether.

Change-Id: Ibf198be9372ce2bec4c8a16d8530edf860733a57
Reviewed-on: https://gerrit.libreoffice.org/65207
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>