Add return types to OnPluginEnd, OnPluginPauseChange, and OnGameFrame

This adds void return types to OnPluginEnd, OnPluginPauseChange, and OnGameFrame.

This allows them to work with `#pragma newdecls required` instead of causing a "function heading differs from prototype" error for them.

**No other changes have been made to sourcemod.inc.**  This is intentional as I think a few design decisions above my level need to be made for this file first.

Edit: Added void return type to timers.inc's OnMapTimeLeftChanged forward as well.