DRTVWR-418: Update forwarding_api.cpp to match current API signature

Specifically, DriverCallback() was accepting several DWORD arguments that
should really be DWORD_PTR arguments. In a 32-bit compile, evidently that was
okay, but for 64 bits they're different sizes.

indra/media_plugins/winmmshim/forwarding_api.cpp

@@ -429,7 +429,7 @@ extern "C" {
 		return DefDriverProc_orig( dwDriverIdentifier, hdrvr, uMsg, lParam1, lParam2);
 	}
 
-	BOOL WINAPI DriverCallback( DWORD dwCallBack, DWORD dwFlags, HDRVR hdrvr, DWORD msg, DWORD dwUser, DWORD dwParam1, DWORD dwParam2)
+	BOOL WINAPI DriverCallback( DWORD_PTR dwCallBack, DWORD dwFlags, HDRVR hdrvr, DWORD msg, DWORD_PTR dwUser, DWORD_PTR dwParam1, DWORD_PTR dwParam2)
 	{
 		ll_winmm_shim_initialize();
 		//OutputDebugString(L"DriverCallback\n");