Add src/main/yang as source path in build-helper-maven-plugin

This is required by yangide so that in-workspace cross-project *.yang
references are correctly resolved; see
https://lists.opendaylight.org/pipermail/yangtools-dev/2016-May/001383.html
thread.

Change-Id: I68b7a6961bb42caf7d48d3bfdca0587cc1c1d1ab
Signed-off-by: Michael Vorburger <vorburger@redhat.com>