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/tests/threadtests.c

@@ -19,7 +19,11 @@
  * otherwise) arising in any way out of the use of this software, even
  * if advised of the possibility of such damage.
  */
+
+#ifndef GLIB_DISABLE_DEPRECATION_WARNINGS
 #define GLIB_DISABLE_DEPRECATION_WARNINGS
+#endif
+
 #include <glib.h>
 #include <glib-object.h>