Fix broken Windows build

TeamCity caught this, but I thought the error was from my warnings
work. Turns out it was a real build failure. Windows doesn’t know which
floating point floor to use. In reality the parentheses are in the
wrong place and should be the floor of the divide by 2. But integer
division is always floor, so it’s not needed at all.

src/ui/QGCParamWidget.cc

@@ -331,7 +331,7 @@ void QGCParamWidget::insertParamAlphabetical(int indexLowerBound, int indexUpper
     }
     else
     {
-        int midpoint = indexLowerBound + floor(indexUpperBound - indexLowerBound)/2;
+        int midpoint = indexLowerBound + ((indexUpperBound - indexLowerBound) / 2);
 
         if (paramItem->text(0).compare(parentItem->child(midpoint)->text(0)) < 0)
         {