Plug memory leak in GTK x-display-monitor-attributes-list

* src/frame.c (free_monitors) [USE_GTK]: Define in the GTK case as
  well.

* src/xfns.c (x-display-monitor-attributes-list) [USE_GTK]: Plug
  memory leak.

* src/frame.h (MonitorInfo): Declare name as pointing to const char.

src/frame.c

@@ -5662,8 +5662,8 @@ selected frame.  This is useful when `make-pointer-invisible' is set.  */)
 
 #ifdef HAVE_WINDOW_SYSTEM
 
-# if (defined HAVE_NS \
-      || (!defined USE_GTK && (defined HAVE_XINERAMA || defined HAVE_XRANDR)))
+# if (defined USE_GTK || defined HAVE_NS || defined HAVE_XINERAMA \
+      || defined HAVE_XRANDR)
 void
 free_monitors (struct MonitorInfo *monitors, int n_monitors)
 {

src/frame.h

@@ -1648,7 +1648,7 @@ flush_frame (struct frame *f)
 struct MonitorInfo {
   XRectangle geom, work;
   int mm_width, mm_height;
-  char *name;
+  const char *name;
 };
 
 extern void free_monitors (struct MonitorInfo *monitors, int n_monitors);

src/xfns.c

@@ -5030,7 +5030,7 @@ Internal use only, use `display-monitor-attributes-list' instead.  */)
       mi->mm_height = height_mm;
 
 #if GTK_CHECK_VERSION (3, 22, 0)
-      mi->name = g_strdup (gdk_monitor_get_model (monitor));
+      mi->name = xstrdup (gdk_monitor_get_model (monitor));
 #elif GTK_CHECK_VERSION (2, 14, 0)
       mi->name = gdk_screen_get_monitor_plug_name (gscreen, i);
 #endif
@@ -5041,6 +5041,11 @@ Internal use only, use `display-monitor-attributes-list' instead.  */)
                                                  primary_monitor,
                                                  monitor_frames,
                                                  source);
+#if GTK_CHECK_VERSION (2, 14, 0)
+  free_monitors (monitors, n_monitors);
+#else
+  xfree (monitors)
+#endif
   unblock_input ();
 #else  /* not USE_GTK */