Move header inclusion guard at the top

GCC has some optimization for the inclusion guard, but they only work if
the check is the outermost one.

We're fairly inconsistent because of historical reasons, so we should
ensure that we follow the same pattern in every public header.

clutter/clutter-transition-group.h

@@ -24,6 +24,10 @@
 #ifndef __CLUTTER_TRANSITION_GROUP_H__
 #define __CLUTTER_TRANSITION_GROUP_H__
 
+#if !defined(__CLUTTER_H_INSIDE__) && !defined(CLUTTER_COMPILATION)
+#error "Only <clutter/clutter.h> can be included directly."
+#endif
+
 #include <clutter/clutter-types.h>
 #include <clutter/clutter-transition.h>