Update linux-headers for applications that use it, like xen.
emerge xen
output:
These are the packages that would be merged, in order: Calculating dependencies ... done! emerge: there are no ebuilds to satisfy ">=sys-kernel/linux-headers-4.11". (dependency required by "app-emulation/xen-tools-4.9.1-r1::nokit" [ebuild]) (dependency required by "app-emulation/xen-4.9.1-r1::nokit" [ebuild]) (dependency required by "xen" [argument])