fix warning

parallel_hashmap/phmap.h

@@ -325,7 +325,7 @@ struct GroupSse2Impl
         return BitMask<uint32_t, kWidth>(
             _mm_movemask_epi8(_mm_sign_epi8(ctrl, ctrl)));
 #else
-        return Match(kEmpty);
+        return Match(static_cast<h2_t>(kEmpty));
 #endif
     }