warnless: suppress compiler warning

If size_t is 32 bits, MSVC warns:
warning C4310: cast truncates constant value
The warning is harmless as CURL_MASK_SCOFFT gets
truncated to the maximum value of size_t.

lib/warnless.c

@@ -183,12 +183,15 @@ curl_off_t curlx_uztoso(size_t uznum)
 #ifdef __INTEL_COMPILER
 #  pragma warning(push)
 #  pragma warning(disable:810) /* conversion may lose significant bits */
+#elif defined(_MSC_VER)
+#  pragma warning(push)
+#  pragma warning(disable:4310) /* cast truncates constant value */
 #endif
 
   DEBUGASSERT(uznum <= (size_t) CURL_MASK_SCOFFT);
   return (curl_off_t)(uznum & (size_t) CURL_MASK_SCOFFT);
 
-#ifdef __INTEL_COMPILER
+#if defined(__INTEL_COMPILER) || defined(_MSC_VER)
 #  pragma warning(pop)
 #endif
 }