After the bssap test in Ie934c5d229140a89763bf2efff86d6a3766cd351, the
subsequent commit Ia2882b7ca31a3219c676986e85045fa08a425d7a was not tested
against the latest head, and its breaking bssap_test was not caught.
Fix current master of osmo-bsc's 'make check' target: add osmo_bsc_mgcp.c and
libosmo-mgcp-client dependencies to bssap_test linkage.
Change-Id: I28719d267452f66d65581c43433e24a9f46cf7dc