Add missing configuration to build-helper-maven-plugin

Add missing configuration to build-helper-maven-plugin

This makes openflowjava work e.g. in Eclipse IDE out-of-the-box (using
https://github.com/vorburger/opendaylight-eclipse-setup configuration),
without any red.  Without this, the paths with the generated code are
not automatically source folder in IDE, so not on classpath, so there's
red errors.

This is how all other projects do it; I just copy/pasted from elsewhere.

PS: In an ideal world, this should be inherited from some parent POM
e.g. in controller, but let's at least already do an ad-hoc fix here.

Change-Id: Ib0b8258de329345422d044f5fea4e9981b67da93
Signed-off-by: Michael Vorburger <vorburger@redhat.com>