libc: improve include usage for exterror sources

Include sys/types.h by exterr.h, since size_t is used.
Drop include of sys/exterr_cat.h, it is useless for the only prototype
provided.

Reviewed by: mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D55337