Add CS_DMG_HEADSHOT define

but i don't know if it works in other games.

( I'm using this already in my only headshot plugin - https://forums.alliedmods.net/showthread.php?t=234169 and works fine in CSS/CSGO )

plugins/include/cstrike.inc

@@ -46,6 +46,8 @@
 #define CS_SLOT_GRENADE		3	/**< Grenade slot (will only return one grenade). */
 #define CS_SLOT_C4			4	/**< C4 slot. */
 
+#define CS_DMG_HEADSHOT		(1 << 30)	/**< Headshot */
+
 enum CSRoundEndReason
 {
 	CSRoundEnd_TargetBombed = 0,           /**< Target Successfully Bombed! */