Log Message:
fcitx5-gtk: Fix linking with libxkbcommon on non-wayland platforms
fcitx5-gtk:s configure step claims that libxkbcommon is optional
but it always links to XKBCommon::XKBCommon. libxbcommon is pulled
in via buildlink3.mk from gtk3 on wayland platforms. On non-wayland
platforms it is not pulled in causing build breakage. Include
libxkbcommons buildlink3.mk in the Makefile to make sure it is
available unconditionally.
No revbump is done in this commit since it is a build fix, all
built packages are already linked to libxkbcommon.