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/des.h b/include/mbedtls/des.h
index 6bfe654..29b96db 100644
--- a/include/mbedtls/des.h
+++ b/include/mbedtls/des.h
@@ -32,6 +32,7 @@
 #else
 #include MBEDTLS_CONFIG_FILE
 #endif
+#include "mbedtls/platform_util.h"
 
 #include <stddef.h>
 #include <stdint.h>