Pacify gcc -Wunused-function on Ubuntu 18.04.3

This improves on 2019-12-25T20:01:[email protected],
by fixing a GCC warning on Ubuntu 18.04.3
“‘x_get_net_workarea’ defined but not used”.
* src/xfns.c (x_get_net_workarea) [!USE_GTK]: Do not define.

Author: eggert

src/xfns.c

@@ -4572,6 +4572,8 @@ On MS Windows, this just returns nil.  */)
     return Qnil;
 }
 
+#ifndef USE_GTK
+
 /* Store the geometry of the workarea on display DPYINFO into *RECT.
    Return false if and only if the workarea information cannot be
    obtained via the _NET_WORKAREA root window property.  */
@@ -4634,8 +4636,6 @@ x_get_net_workarea (struct x_display_info *dpyinfo, XRectangle *rect)
   return result;
 }
 
-#ifndef USE_GTK
-
 /* Return monitor number where F is "most" or closest to.  */
 static int
 x_get_monitor_for_frame (struct frame *f,