gobject: Use _ prefixing in private headers over G_GNUC_INTERNAL in public headers

This helps out gtk-doc and g-ir-scanner, and also makes much clearer
what's private versus public.

https://bugzilla.gnome.org/show_bug.cgi?id=651745

gobject/gvaluetransform.c

@@ -22,6 +22,7 @@
 #include <string.h>
 
 #include "gvalue.h"
+#include "gtype-private.h"
 #include "genums.h"
 
 
@@ -243,7 +244,7 @@ value_transform_flags_string (const GValue *src_value,
 /* registration
  */
 void
-g_value_transforms_init (void)
+_g_value_transforms_init (void)
 {
   /* some transformations are a bit questionable,
    * we currently skip those