Changes

jobs/gerrit-verifications: osmo-ttcn3-hacks: do make clean

It's easy to break the build verification [for everybody] by
submitting a patch that creates a symlink to a non-existent file.
The workspace is not wiped, so the stale symlink remains and results
in failures like this:

(cd sgsn && ./gen_links.sh && ./regen_makefile.sh)
ttcn3_makefilegen: error: Cannot find any source file for argument `NAS_Templates.ttcn'.
make: *** [Makefile:131: _build/sgsn/Makefile] Error 1

Let's invoke the 'clean' recipe before 'compile' to avoid this.

Change-Id: I886a99e7b1e8c42ccc7441685ecd7b2c54d5a7af