This is not a build time dependency (same as sys-apps/pciutils). Build system generates loader for it, which uses dlopen and dlsym for accessing udev functionality.
In overlay: https://gitweb.gentoo.org/proj/qt.git/commit/?id=ba51bab012d9fb357f3e78204a197656cd1c6485
Thanks, fixed in git. https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=90f45d0172ad22d6b8a6e77025499156b704dcc9