gtkcomposetable.c: Fix build on non-X11

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.

gtk/gtkcomposetable.c

@@ -25,6 +25,7 @@
 
 #include "gtkcomposetable.h"
 #include "gtkimcontextsimple.h"
+#include "gtkprivate.h"
 
 
 #define GTK_COMPOSE_TABLE_MAGIC "GtkComposeTable"