autotools: don't ship the git_sha1.h generated in git in the tarballs

autotools: don't ship the git_sha1.h generated in git in the tarballs

This file is regenerated at build time anyway, so this would just get
overwritten anyway. No reason to ship it in the tarball.

Fixes: 44df06211cf2c301f6ef "autotools: include git_sha1.h in dist tarball"
Fixes: 471f708ed6f4787813d0 "git_sha1: simplify logic"
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>