tzfile.5: catch up to r204333

The stdtime sources were moved from lib/libc to contrib/tzcode, and tzfile.h
is not an installed header, so the man page refers to its location in the
source tree.

The documentation could be more clear about the internal nature of the
header, but those changes should go through upstream tzcode.

PR:	docs/176864
Approved by:	hrs (mentor)