ap: Fix build issue with missing _GNU_SOURCE

CC       src/ap.o
src/ap.c: In function ‘ap_reset’:
src/ap.c:286:9: error: implicit declaration of function ‘explicit_bzero’ [-Wimplicit-function-declaration]
  286 |         explicit_bzero(ap->passphrase, sizeof(ap->passphrase));
      |         ^~~~~~~~~~~~~~

src/ap.c

@@ -24,6 +24,7 @@
 #include <config.h>
 #endif
 
+#define _GNU_SOURCE
 #include <errno.h>
 #include <linux/if_ether.h>
 #include <netinet/in.h>