Fix gcc -O3 warnings

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/gcm.c b/library/gcm.c
index 90c1d1d..976d6d7 100644
--- a/library/gcm.c
+++ b/library/gcm.c
@@ -412,8 +412,17 @@
         while (iv_len > 0) {
             use_len = (iv_len < 16) ? iv_len : 16;
 
+#if defined(MBEDTLS_COMPILER_IS_GCC)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic warning "-Wstringop-overflow=0"
+#endif
+
             mbedtls_xor(ctx->y, ctx->y, p, use_len);
 
+#if defined(MBEDTLS_COMPILER_IS_GCC)
+#pragma GCC diagnostic pop
+#endif
+
             gcm_mult(ctx, ctx->y, ctx->y);
 
             iv_len -= use_len;