Mark DataPacks as references in callback definitions

[strafe]

In 98f1126 the callback definitions were updated to transitional syntax. However, the ampersand indicating the DataPacks are passed by reference was dropped (I assume accidentally).

In their current state, no plugins are able to use these natives as you'll receive an error when trying to manipulate the Handle:

L 01/04/2021 - 02:17:16: [SM] Exception reported: Handle 18f8 cannot be cloned because it is invalid (error 4)
L 01/04/2021 - 02:17:16: [SM] Blaming: plugin.smx
L 01/04/2021 - 02:17:16: [SM] Call stack trace:
L 01/04/2021 - 02:17:16: [SM]   [0] CloneHandle
L 01/04/2021 - 02:17:16: [SM]   [1] Line 221, plugin.sp::QueryGameMEStatsReceived
L 01/04/2021 - 02:17:16: [SM]   [3] Call_Finish
L 01/04/2021 - 02:17:16: [SM]   [4] Line 4076, gameme.sp::gameme_raw_messag

This happens because the Handle is pushed with Call_PushCellRef rather than Call_PushCell, therefore requiring the prepended ampersand:

plugin-sourcemod/scripting/gameme.sp

Lines 3888 to 3895 in 98f1126

	Call_StartFunction(data.callback_data_plugin, data.callback_data_function);
	Call_PushCell(RAW_MESSAGE_CALLBACK_PLAYER);
	Call_PushCell(data.callback_data_payload);
	Call_PushCell(client);
	Call_PushCellRef(pack);
	Call_Finish(_:result);