Added GetSteamAccountID function to IPlayerHelpers and native for sp (bug 5548, r=asherkin).

plugins/include/clients.inc

@@ -269,6 +269,18 @@ native bool:GetClientIP(client, String:ip[], maxlen, bool:remport=true);
  */
 native bool:GetClientAuthString(client, String:auth[], maxlen, bool:validate=true);
 
+/**
+ * Returns the client's Steam account ID.
+ *
+ * @param client		Client Index.
+ * @param validate		Check backend validation status.
+ * 				DO NOT PASS FALSE UNLESS YOU UNDERSTAND THE CONSEQUENCES,
+ *			        You WILL KNOW if you need to use this, MOST WILL NOT.
+ * @return				Steam account ID or 0 if not available.
+ * @error				If the client is not connected or the index is invalid.
+ */
+native GetSteamAccountID(client, bool:validate=true);
+
 /**
  * Retrieves a client's user id, which is an index incremented for every client
  * that joins the server.