This dependency is a nuisance if one wants to install a Wayland system without X11. Since Weston depends on xkeyboard-config, this will necessarily pull in X, although xkeyboard-config would work without its dependency. For reference, an exlib where this is correct: https://git.exherbo.org/x11.git/tree/packages/x11-apps/xkeyboard-config/xkeyboard-config.exlib
In fact, xkbcomp should have RDEPEND=""
The dependency was added due to bug 375547. It might be worth checking if the situation has changed in the meantime.
commit e66751674fba6abf1926db5a1fec6e75cdb8b2fe Author: Chí-Thanh Christopher Nguyễn <chithanh@gentoo.org> Date: Sat Jun 4 11:41:47 2016 +0200 x11-misc/xkeyboard-config: Version bump Also block incompatible libX11 and xkbcomp instead of depending on them. Bug: https://bugs.gentoo.org/show_bug.cgi?id=583184 Bug: https://bugs.gentoo.org/show_bug.cgi?id=584958