Fix compilation with GTK versions older than 3

* src/xfns.c (x_get_net_workarea): Change a recently moved #ifndef
so that GTK builds which need this function will compile it.
Reported by John <jpff@codemist.co.uk>.

(cherry picked from commit d36adb544d)

src/xfns.c

@@ -4572,7 +4572,7 @@ On MS Windows, this just returns nil.  */)
     return Qnil;
 }
 
-#ifndef USE_GTK
+#if !defined USE_GTK || !defined HAVE_GTK3
 
 /* Store the geometry of the workarea on display DPYINFO into *RECT.
    Return false if and only if the workarea information cannot be