tls: Move variable sized tls_record_t struct to end of tls_t data

clang complains about the the non-last variable length member.

src/libtls/tls.c

@@ -171,15 +171,15 @@ struct private_tls_t {
 	 */
 	size_t outpos;
 
-	/**
-	 * Partial TLS record header received
-	 */
-	tls_record_t head;
-
 	/**
 	 * Position in partially received record header
 	 */
 	size_t headpos;
+
+	/**
+	 * Partial TLS record header received
+	 */
+	tls_record_t head;
 };
 
 /**