Fix build failure with DOCS option and edge case

In certain situations, e.g. when docbook ports have been deinstalled
before building audacity with DOCS option, build may fail. Prevent
this case by explicitly including required docbook ports in
DOCS_BUILD_DEPENDS

PR: 204785
Submitted by: rhurlin@gwdg.de
Reviewed by: xxjack12xx@gmail.com (maintainer)
Approved by: xxjack12xx@gmail.com (maintainer)