gtk.c: Fix memory leak in gd_set_keycode_type()

 this memory leak is introduced by the original
 commit 3158a348

 valgrind out showing:
 ==14553== 21,459 (72 direct, 21,387 indirect) bytes in 1 blocks are definitely
 lost in loss record 8,055 of 8,082
 ==14553==    at 0x4A06BC3: calloc (vg_replace_malloc.c:618)
 ==14553==    by 0x80DBFBC: XkbGetKeyboardByName (in /usr/lib64/libX11.so.6.3.0)
 ==14553==    by 0x40C704: gtk_display_init (gtk.c:1798)
 ==14553==    by 0x1AEDC1: main (vl.c:4480)
Signed-off-by: NChen Fan <chen.fan.fnst@cn.fujitsu.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/gtk.c

@@ -1810,6 +1810,13 @@ static void gd_set_keycode_type(GtkDisplayState *s)
             fprintf(stderr, "unknown keycodes `%s', please report to "
                     "qemu-devel@nongnu.org\n", keycodes);
         }
+
+        if (desc) {
+            XkbFreeKeyboard(desc, XkbGBN_AllComponentsMask, True);
+        }
+        if (keycodes) {
+            XFree(keycodes);
+        }
     }
 #endif
 }