New macro MBEDTLS_CHECK_RETURN

Put this macro before a function declaration to indicate that its result
must be checked. This commit supports GCC-like compilers and MSVC >=2012.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/aes.h b/include/mbedtls/aes.h
index ab8793c..a74e413 100644
--- a/include/mbedtls/aes.h
+++ b/include/mbedtls/aes.h
@@ -45,6 +45,7 @@
 #else
 #include MBEDTLS_CONFIG_FILE
 #endif
+#include "mbedtls/platform_util.h"
 
 #include <stddef.h>
 #include <stdint.h>