Define _GNU_SOURCE to get PTHREAD_MUTEX_INITIALIZER

Fixes
| zmalloc.c:87:37: error: 'PTHREAD_MUTEX_DEFAULT' undeclared here (not in a function)
|    87 | pthread_mutex_t used_memory_mutex = PTHREAD_MUTEX_INITIALIZER;
|       |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>
--- a/src/zmalloc.c
+++ b/src/zmalloc.c
@@ -28,6 +28,7 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>