epa-peap: Fix build issue with missing _GNU_SOURCE

CC       src/eap-peap.o
src/eap-peap.c: In function ‘eap_peap_phase2_complete’:
src/eap-peap.c:120:9: error: implicit declaration of function ‘explicit_bzero’ [-Wimplicit-function-declaration]
  120 |         explicit_bzero(peap_state->key, sizeof(peap_state->key));
      |         ^~~~~~~~~~~~~~

src/eap-peap.c

@@ -24,6 +24,7 @@
 #include <config.h>
 #endif
 
+#define _GNU_SOURCE
 #include <string.h>
 #include <stdio.h>
 #include <errno.h>