zlib: Fix bootstrap build on macOS

gzlib.c uses lseek but does not include unistd.h, where it is defined to
live. On FreeBSD this happens to work due to sys/types.h declaring it,
but on macOS it fails due to being only implicitly declared, which is
treated as an error by default by the system Clang.

MFC after: 1 week

(cherry picked from commit 6010a892b4258e90037dabf9765066d07b86f13f)