silence a msvc build warning

implicit double -> float conversion; avoiding sqrtf (c99) for
compatibility

Change-Id: If9b367dde237fc5173c1445cc60b16aa6ad2821c

src/utils/quant_levels.c

@@ -140,7 +140,7 @@ int QuantizeLevels(uint8_t* data, int width, int height,
 
   // Compute final mean squared error if needed.
   if (mse) {
-    *mse = sqrt(err);
+    *mse = (float)sqrt(err);
   }
 
   return 1;