glib: Avoid redefining GLIB_DISABLE_DEPRECATION_WARNINGS

It may be defined by the environment (we document that as being allowed)
— if so, individual files should not try to redefine it, as that causes
a preprocessor warning.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

gobject/gboxed.c

@@ -20,7 +20,9 @@
 #include <string.h>
 
 /* for GValueArray */
+#ifndef GLIB_DISABLE_DEPRECATION_WARNINGS
 #define GLIB_DISABLE_DEPRECATION_WARNINGS
+#endif
 
 #include "gboxed.h"
 #include "gclosure.h"