[rpm] Fix wrong permissions for initfs.tar.bz2

rpmlint found executable permissions for initfs.tar.bz2. Fixed by
explicitly setting the permissions to 644 when installing.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jolla.com>
Merge pull request #2 from kjokinie/perm-fix

[rpm] Fix wrong permissions for initfs.tar.bz2