docs build: don't clober noinst_DATA
docs build: don't clober noinst_DATA

Use a += instead of a = assignment, since this value is also set from
other places.

cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit