Cleanup: use the system-handle arg for GHOST_SetMultitouchGestures

There was an unused argument warning, quiet by using the argument.

intern/ghost/intern/GHOST_C-api.cpp

@@ -745,7 +745,7 @@ GHOST_TSuccess GHOST_InvalidateWindow(GHOST_WindowHandle windowhandle)
 
 void GHOST_SetMultitouchGestures(GHOST_SystemHandle systemhandle, const bool use)
 {
-  GHOST_ISystem *system = GHOST_ISystem::getSystem();
+  GHOST_ISystem *system = (GHOST_ISystem *)systemhandle;
   return system->setMultitouchGestures(use);
 }