gtk.c: Fix memory leak in gd_set_keycode_type()

 this memory leak is introduced by the original
 commit 3158a3482b0093e41f2b2596fba50774ea31ae08

 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: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
