update .gitignore for install_manifest.txt
update .gitignore for install_manifest.txt

ignore it.

Signed-off-by: Andy Grover <b7d524d2f5cc5aebadb6b92b08d3ab26911cde33@redhat.com>

agrover/tcmu-runner,JefferyHu/tcmu-runner,superlibj/tcmu-runner,liuy/tcmu-runner,open-iscsi/tcmu-runner,open-iscsi/tcmu-runner,yasker/tcmu-runner,lxbsz/tcmu-runner