gnutls: fixed compilation against versions < 2.12.0

The AES-GCM ciphers were added to GnuTLS as late as ver. 3.0.1 but
the code path in which they're referenced here is only ever used for
somewhat older GnuTLS versions. This caused undeclared identifier errors
when compiling against those.

RELEASE-NOTES

@@ -40,6 +40,7 @@ This release includes the following bugfixes:
  o nss: fix a memory leak when CURLOPT_CRLFILE is used
  o gnutls: ignore invalid certificate dates with VERIFYPEER disabled
  o gnutls: fix SRP support with versions of GnuTLS from 2.99.0
+ o gnutls: fixed compilation against versions < 2.12.0
  o 
 
 This release includes the following known bugs:

lib/vtls/gtls.c

@@ -369,10 +369,17 @@ gtls_connect_step1(struct connectdata *conn,
   struct in_addr addr;
 #endif
 #ifndef USE_GNUTLS_PRIORITY_SET_DIRECT
-  static int cipher_priority[] = { GNUTLS_CIPHER_AES_128_GCM,
-    GNUTLS_CIPHER_AES_256_GCM, GNUTLS_CIPHER_AES_128_CBC,
-    GNUTLS_CIPHER_AES_256_CBC, GNUTLS_CIPHER_CAMELLIA_128_CBC,
-    GNUTLS_CIPHER_CAMELLIA_256_CBC, GNUTLS_CIPHER_3DES_CBC,
+  static const int cipher_priority[] = {
+  /* These two ciphers were added to GnuTLS as late as ver. 3.0.1,
+     but this code path is only ever used for ver. < 2.12.0.
+     GNUTLS_CIPHER_AES_128_GCM,
+     GNUTLS_CIPHER_AES_256_GCM,
+  */
+    GNUTLS_CIPHER_AES_128_CBC,
+    GNUTLS_CIPHER_AES_256_CBC,
+    GNUTLS_CIPHER_CAMELLIA_128_CBC,
+    GNUTLS_CIPHER_CAMELLIA_256_CBC,
+    GNUTLS_CIPHER_3DES_CBC,
   };
   static const int cert_type_priority[] = { GNUTLS_CRT_X509, 0 };
   static int protocol_priority[] = { 0, 0, 0, 0 };