Qt: Use std::abs instead of abs

...since <cmath> is included, not <math.h>. May or may not fix
https://bugs.dolphin-emu.org/issues/10906

Source/Core/DolphinQt2/Config/Mapping/MappingIndicator.cpp

@@ -171,7 +171,7 @@ void MappingIndicator::DrawStick()
   // Emulated cursor position
   float virt_curx, virt_cury;
 
-  if (abs(curx) < deadzone && abs(cury) < deadzone)
+  if (std::abs(curx) < deadzone && std::abs(cury) < deadzone)
   {
     virt_curx = virt_cury = 0;
   }