build: build java artifacts before processing modules

Currently files which are part of 'java' module are built as part of
the main osv build. These files need to be ready before the module is
processed because the module relies on them.

Because we didn't have explicit dependency between processing modules
and building java artifacts the build failed when parallel jobs were
used. The proper fix is to move all java stuff to modules/java so that it
would be built as part of the module build. Before that happens here's
a quick fix which adds explicit dependency to build.mk.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>