eap-wsc: Fix build issue with missing _GNU_SOURCE

CC       src/eap-wsc.o
src/eap-wsc.c: In function ‘encrypted_settings_decrypt’:
src/eap-wsc.c:258:9: error: implicit declaration of function ‘explicit_bzero’ [-Wimplicit-function-declaration]
  258 |         explicit_bzero(decrypted, encrypted_len);
      |         ^~~~~~~~~~~~~~

src/eap-wsc.c

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