[build] Check if git index actually exists

If iPXE is used as a git submodule then the ../.git/index file will
not exist, and the build will fail.  Fix by checking that the git
index file exists before adding it as a build dependency.

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>