libosmo-legacy-mgcp: fix link against libgsm

libosmo-legacy-mgcp was not linked against libgsm
when built with --enable-mgcp-transcoding.
When afterwards a binary such as osmo-bsc is built and
tries to link against it, it will fail with an error
like the one below:
/home/pespin/dev/sysmocom/bin/../build/new/out/lib/libosmo-legacy-mgcp.so: undefined reference to `gsm_create'

Tested that building with this patch fixes the issue.
Also tested that it still builds fine without --enable-mgcp-transcoding.

Change-Id: I5ed356ac0c8f476e263fc6dcc5613d594890dfcd
1 file changed