cmake: mgr: exclude .gitignore

cmake: mgr: exclude .gitignore

Fixes RPMLINT warning:

ceph-mgr.x86_64: W: version-control-internal-file /usr/lib64/ceph/mgr/.gitignore
ceph-mgr.x86_64: W: version-control-internal-file /usr/lib64/ceph/mgr/dashboard/static/AdminLTE-2.3.7/.gitignore
You have included file(s) internally used by a version control system in the
package. Move these files out of the package and rebuild it.

Note: the backslash has to be doubled up for the regex to make
it through CMake.

Signed-off-by: Nathan Cutler <ncutler@suse.com>