fix createCallback complaining on client side

shared/callback.lua

@@ -13,6 +13,7 @@
 --- end)
 --- ```
 function createCallback(callbackName, funct)
+    if isServer() then
         if isStarted(OXLibExport) then
             lib.callback.register(callbackName, funct)
         else
@@ -31,6 +32,7 @@ function createCallback(callbackName, funct)
             end
         end
     end
+end
 
 --- Triggers a server callback and returns the result.
 ---