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>

glib/tests/unicode.c

@@ -23,7 +23,9 @@
  */
 
 /* We are testing some deprecated APIs here */
+#ifndef GLIB_DISABLE_DEPRECATION_WARNINGS
 #define GLIB_DISABLE_DEPRECATION_WARNINGS
+#endif
 
 #include <locale.h>