sys/acl.h: reduce header pollution

In commit d1dfd921774f0 the acl allocator was switched from malloc(9) to
uma(9) and done in a way required vm/uma.h and prerequisites. This added
considerable header pollution to sys/sysproto.h. The uma details were
hidden in commit b998d381f2868, but the header pollution remained. Add
less broad includes as required to keep the header self contained.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44468