astro/osmium: unbreak build with DOXYGEN=on after r394778

NO_BUILD does more than dummy do-build. It creates BUILD_COOKIE that marks
all targets in _BUILD_SEQ as done, including do-build-${opt}-on/off and
post-build-${opt}-on/off. Since the targets haven't been run yet
do-install-DOXYGEN-on fails due to missing files.

Fix by leaking a call to nop "all" target in vendor Makefile. It should
be less kludgy than defining our own dummy do-build and less confusing
than treating do-build itself as an option helper. In case "all" starts
doing something useful the port would automatically take advantage of it.

PR: 202951
Reviewed by: mat
Approved by: portmgr blanket