Changes

testenv: osmo-dev: build with gtp_linux.opts

Run the osmo-ggsn ./configure with --enable-gtp-linux, in preparation
for testing kernel gtp-u with testenv too.

Related: https://gitea.osmocom.org/osmocom/osmo-dev/src/branch/master/gtp_linux.opts
Change-Id: Ia8ea5494597e0fedae93f501c8bc7d6115543f36