Redmine#6067: install .gitignore explicitly

see https://dev.cfengine.com/issues/6067
Alternative (in case we ever ship sub-dirs with .gitignore in them) would be, just before this for loop, to
`if [ -f $$d/.gitignore ]; then $(INSTALL_DATA) $$d/.gitignore $(DESTDIR)/$$d/.gitignore; fi`