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.h

@@ -41,7 +41,7 @@ public:
 
   std::string GetGUIName() const { return m_gui_name; }
   std::string GetProfileName() const { return m_profile_name; }
-  std::size_t GetControllerCount() const;
+  int GetControllerCount() const;
 
   // These should be used after creating all controllers and before clearing them, respectively.
   void RegisterHotplugCallback();