Include <unistd.h> for lseek()

Include <unistd.h> for lseek()

This unbreaks my build in OpenBSD.  The old `long lseek()` conflicts
with `off_t lseek()` in OpenBSD headers, because long and off_t are
different types.  Commit b4df26e caused "system.h" to include some
headers where OpenBSD declares lseek().

Manuals for lseek() say to #include <unistd.h>.  Do so to be portable
to systems where other headers don't declare lseek().