chrome-icu: add dependencies related to xkbcommon USE.

This CL adds the dependencies to RDEPEND when USE flag "xkbcommon" is
enabled. This is identical to chromeos-chrome.ebuild, which will make
CQ more likely to catch potential errors of chrome-icu.ebuild by
testing chromeos-chrome.ebuild related to libxkbcommon.

BUG=none
TEST="emerge-nocturne chrome-icu" works.

Change-Id: Ibea728a7d4bf9b7324945306e909b8625300dcb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2442914
Tested-by: Honglin Yu <honglinyu@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Honglin Yu <honglinyu@chromium.org>
1 file changed