do not use stat.st_mtime of a non-regular file

do not use stat.st_mtime of a non-regular file

* gzip.c: Include "timespec.h".
(treat_stdin): Use st_mtime only from a regular file.
This matters at least on Cygwin 1.7.1-1, for which a stdin-pipe has
the mtime of /dev/null, rather than the gzip-documented-for-pipes
"current time".  Reported by Denis Excoffier.