LDAP: fix compile warning

Sometimes the compile will emit the following warning, so we'll just avoid it by making it call a function in the LDAP module which will never have this problem.

warning: :GenServer.call/2 is undefined (module :GenServer is not available or is yet to be defined)

lib/pleroma/ldap.ex

@@ -94,6 +94,10 @@ defmodule Pleroma.LDAP do
     :ok
   end
 
+  def bind_user(name, password) do
+    GenServer.call(__MODULE__, {:bind_user, name, password})
+  end
+
   defp connect do
     ldap = Config.get(:ldap, [])
     host = Keyword.get(ldap, :host, "localhost")