eap-tls: Fix build issue with missing _GNU_SOURCE

CC       src/eap-tls.o
src/eap-tls.c: In function ‘eap_tls_tunnel_ready’:
src/eap-tls.c:56:9: error: implicit declaration of function ‘explicit_bzero’ [-Wimplicit-function-declaration]
   56 |         explicit_bzero(msk_emsk, sizeof(msk_emsk));
      |         ^~~~~~~~~~~~~~
src/eap-tls.c: In function ‘eap_tls_check_keys_match’:
src/eap-tls.c:82:21: error: implicit declaration of function ‘alloca’; did you mean ‘calloc’? [-Wimplicit-function-declaration]
   82 |         encrypted = alloca(size);
      |                     ^~~~~~
      |                     calloc
src/eap-tls.c:82:19: error: assignment to ‘uint8_t *’ {aka ‘unsigned char *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
   82 |         encrypted = alloca(size);
      |                   ^
src/eap-tls.c:83:19: error: assignment to ‘uint8_t *’ {aka ‘unsigned char *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
   83 |         decrypted = alloca(size);
      |                   ^

src/eap-tls.c

@@ -24,9 +24,11 @@
 #include <config.h>
 #endif
 
+#define _GNU_SOURCE
 #include <string.h>
 #include <stdio.h>
 #include <errno.h>
+#include <alloca.h>
 #include <ell/ell.h>
 
 #include "src/missing.h"