Makefile.include: fix missing target for libraries

Unittests add libraries in 'BASELIBS' which do not have any rules to be built as
they are built by 'application.inc.mk', packages and the DIRS variable.
So make complains about missing target for the unittests archives.

The fix tells these files are generated when building '$(APPLICATION_MODULE).a'.

The bug was introduced by #8844

Fixes #8910