gtkcomposetable.c: Fix build on non-X11
Hi,
From the commit message:
The build breaks with a C4013 warning/error on Visual Studio because we don't have a prototype defined for _gtk_get_datadir(), so include gtkprivate.h.
The vs2017-x64 CI did not catch this error because it is building GLib as a fallback subproject, causing the msvc_recommended_pragmas.h header not to be found, which is used to detect problems like this.
With blessings, thank you!