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.
#include "gtkcomposetable.h"
#include "gtkimcontextsimple.h"
+#include "gtkprivate.h"
#define GTK_COMPOSE_TABLE_MAGIC "GtkComposeTable"