[cogl-debug] avoid warnings when using COGL_NOTE with unbraced else clauses

When not building a debug build the compiler was warning about empty
else clauses with no braces due to code like:
  if (blah)
	do_foo();
  else
	COGL_NOTE (DRAW, "a-wibble");

This simply ensures that even for non debug builds COGL_NOTE will expand to
a single statement.

cogl-debug.h

@@ -67,7 +67,7 @@ typedef enum {
 
 #else /* !COGL_ENABLE_DEBUG */
 
-#define COGL_NOTE(type,...)
+#define COGL_NOTE(type,...) G_STMT_START {} G_STMT_END
 
 #endif /* COGL_ENABLE_DEBUG */