Restore scripts/make_gitrev.sh

Commit 109d2c931020 ("Move zfs_gitrev.h to build directory") removed
scripts/make_gitrev.sh, putting the logic into the Makefile itself.

However, at least the Arch Linux packager wants the script so that the
file can be generated without having to run configure first, for
DKMS packaging purposes.

So move the make recipe back into the script.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10595