md5_helper.h: fix type conversion warning

add a cast in Add() from size_t -> uint32; quiets a visual studio build
warning

Change-Id: I0d87a3e460faf1fe2d4fb44df5f4042ef7500190

test/md5_helper.h

@@ -43,7 +43,7 @@ class MD5 {
   }
 
   void Add(const uint8_t *data, size_t size) {
-    MD5Update(&md5_, data, size);
+    MD5Update(&md5_, data, static_cast<uint32_t>(size));
   }
 
   const char *Get(void) {