Stop duplicating <linux/capabilities.h>.

This doesn't help the person who wants CAP_BPF, but we can fix that
better by adding it to our stale glibc (and our future switch to musl
will mean we'll never have similar problems again).

I think this just dates from when we still supported building the OS on
darwin, but those days are long gone, and I think this hack can be put
to rest now...

Test: treehugger
Change-Id: I3c2e56c68a5b00c1ad0aed422c6ce60886063f50