dns/bind918: MFH: unbreak build with docs excluded

This is correction after latest mat's change to make it really work.
Default build unaffected, so PORTREVISON not bumped.

PR: 262773
Approved by: mat (maintainer timeout, 2 weeks)
Submitted by: Michael Osipov
Requested by: many

(cherry picked from commit d22c2932f114852aaaddd4f2f1492a8ba3e1218c)