Update SourcePawn.

This brings in a few breaking changes.

One, INVALID_FUNCTION is now 0 instead of -1. This is long overdue.
Plugins should transparently work except in two cases:

  1. Third-party extensions that have a hardcoded test for -1 will no
     longer work. A new API has been provided for this,
     GetFunctionByIdOrNull.
  2. If a plugin "framework" uses INVALID_FUNCTION anywhere in its
     exported API, then all plugins using that framework need to be
     recompiled together, so they agree on the value of
     INVALID_FUNCTION.

Hopefully the damage here is minimal. The core plugin version has been
bumped to 7 to try and limit conflicts.

Second, braceless functions are no longer supported. There wasn't really
any way around this and it's better to bite the bullet now. This affects
source compatibility, but not binary compatibility.

Third, the "using" keyword is no longer implemented. SourceMod now has a
Handle methodmap again. Plugins compiled against this new methodmap will
require a "Handle.~Handle" native, which 1.12 now provides.

extensions/clientprefs/natives.cpp

@@ -398,7 +398,7 @@ cell_t AddSettingsPrefabMenuItem(IPluginContext *pContext, const cell_t *params)
 
 
 	/* User passed a function id for a callback */
-	if (params[4] != -1)
+	if (!pContext->IsNullFunctionId(params[4]))
 	{
 		pItem->forward = forwards->CreateForwardEx(NULL, ET_Ignore, 5, NULL, Param_Cell, Param_Cell, Param_Cell, Param_String, Param_Cell); 
 		pItem->forward->AddFunction(pContext, static_cast<funcid_t>(params[4]));