FreeLookController: Fix signed/unsigned warning

Loop index int i was being compared against GetControllerCount() which
returned a size_t.  This was the only place GetControllerCount() was
called from so the change of return type doesn't disturb anything else.

Changing the loop index to size_t wouldn't work as well since it's
passed into GetController(), which takes an int and is called from many
places, so it would need a cast anyway on an already busy line.

Source/Core/InputCommon/InputConfig.cpp

@@ -186,9 +186,9 @@ bool InputConfig::ControllersNeedToBeCreated() const
   return m_controllers.empty();
 }
 
-std::size_t InputConfig::GetControllerCount() const
+int InputConfig::GetControllerCount() const
 {
-  return m_controllers.size();
+  return static_cast<int>(m_controllers.size());
 }
 
 void InputConfig::RegisterHotplugCallback()