Tab-indent Gnttab to match Gntshr

$ camlp4o -printer o -no_comments lib/gnt.ml | md5sum
385ea8ad2bad3c03f3748d9c7a1a94f7  -
$ git checkout HEAD^
$ camlp4o -printer o -no_comments lib/gnt.ml | md5sum
385ea8ad2bad3c03f3748d9c7a1a94f7  -

Signed-off-by: John Else <john.else@citrix.com>
Add with_gnttab and with_gntshr

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #38 from johnelse/with-gnttab-gntshr

Add with_gnttab and with_gntshr