Add __attribute__ ((nonstring)) to remove unterminated-string-initialization warning

Signed-off-by: Felix Conway <felix.conway@arm.com>
diff --git a/library/ssl_tls13_keys.c b/library/ssl_tls13_keys.c
index fd559a7..dcbfcf0 100644
--- a/library/ssl_tls13_keys.c
+++ b/library/ssl_tls13_keys.c
@@ -82,7 +82,8 @@
  *            the HkdfLabel structure on success.
  */
 
-static const char tls13_label_prefix[6] = "tls13 ";
+/* We need to tell the compiler that we meant to leave out the null character. */
+static const char tls13_label_prefix[6] __attribute__ ((nonstring)) = "tls13 ";
 
 #define SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN(label_len, context_len) \
     (2                     /* expansion length           */ \
diff --git a/library/ssl_tls13_keys.h b/library/ssl_tls13_keys.h
index 14f6e48..f6d02b5 100644
--- a/library/ssl_tls13_keys.h
+++ b/library/ssl_tls13_keys.h
@@ -40,8 +40,9 @@
 
 #if defined(MBEDTLS_SSL_PROTO_TLS1_3)
 
+/* We need to tell the compiler that we meant to leave out the null character. */
 #define MBEDTLS_SSL_TLS1_3_LABEL(name, string)       \
-    const unsigned char name    [sizeof(string) - 1];
+    const unsigned char name    [sizeof(string) - 1] __attribute__ ((nonstring));
 
 union mbedtls_ssl_tls13_labels_union {
     MBEDTLS_SSL_TLS1_3_LABEL_LIST