Fix gcc compile warnings

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/platform_util.c b/library/platform_util.c
index d4574f4..06c7820 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -130,10 +130,19 @@
 #if defined(__GNUC__)
         /* For clang and gcc, pretend that we have some assembly that reads the
          * zero'd memory as an additional protection against being optimised away. */
+#if defined(__clang__)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wvla"
+#elif defined(MBEDTLS_COMPILER_IS_GCC)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wvla"
+#endif
         asm volatile ("" : : "m" (*(char (*)[len]) buf) : );
+#if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(MBEDTLS_COMPILER_IS_GCC)
+#pragma GCC diagnostic pop
+#endif
 #endif
     }
 }