Cvars: Add OnCvarChanged global forward

I really tried to use "cvar_changed" as name, but it bothered me so much to look that weird I could not keep it.

plugins/include/amxmodx.inc

@@ -29,6 +29,7 @@
 #include <newmenus>
 #include <textparse_smc>
 #include <textparse_ini>
+#include <cvars>
 
 /**
  * Called just after server activation.

plugins/include/cvars.inc

@@ -0,0 +1,23 @@
+// vim: set ts=4 sw=4 tw=99 noet:
+//
+// AMX Mod X, based on AMX Mod by Aleksander Naszko ("OLO").
+// Copyright (C) The AMX Mod X Development Team.
+//
+// This software is licensed under the GNU General Public License, version 3 or higher.
+// Additional exceptions apply. For full license details, see LICENSE.txt or visit:
+//     https://alliedmods.net/amxmodx-license
+
+#if defined _cvars_included
+	#endinput
+#endif
+#define _cvars_included
+
+/**
+ * Called when a console variable's value is changed.
+ * 
+ * @param cvarHandle    Handle to the cvar that was changed
+ * @param oldValue      String containing the value of the cvar before it was changed
+ * @param newValue      String containing the new value of the cvar
+ * @param cvarName      String containing the name of the cvar
+ */
+forward OnCvarChanged(cvarHandle, const oldValue[], const newValue[], const cvarName[]);