libosmo-legacy-mgcp: Link against libosmovty, don't link libosmogsm

This fixes the following dpkg-shlibdeps warnings:

Change-Id: I648bbda50520808afcf2a6ce64fe710df918936c
dpkg-shlibdeps: warning: symbol install_element_ve used by debian/libosmo-legacy-mgcp0/usr/lib/x86_64-linux-gnu/libosmo-legacy-mgcp.so.0.0.0 found in none of the libraries
dpkg-shlibdeps: warning: symbol argv_concat used by debian/libosmo-legacy-mgcp0/usr/lib/x86_64-linux-gnu/libosmo-legacy-mgcp.so.0.0.0 found in none of the libraries
dpkg-shlibdeps: warning: symbol vty_out used by debian/libosmo-legacy-mgcp0/usr/lib/x86_64-linux-gnu/libosmo-legacy-mgcp.so.0.0.0 found in none of the libraries
dpkg-shlibdeps: warning: symbol vty_install_default used by debian/libosmo-legacy-mgcp0/usr/lib/x86_64-linux-gnu/libosmo-legacy-mgcp.so.0.0.0 found in none of the libraries
dpkg-shlibdeps: warning: symbol vty_read_config_file used by debian/libosmo-legacy-mgcp0/usr/lib/x86_64-linux-gnu/libosmo-legacy-mgcp.so.0.0.0 found in none of the libraries
dpkg-shlibdeps: warning: symbol install_element used by debian/libosmo-legacy-mgcp0/usr/lib/x86_64-linux-gnu/libosmo-legacy-mgcp.so.0.0.0 found in none of the libraries
dpkg-shlibdeps: warning: symbol install_node used by debian/libosmo-legacy-mgcp0/usr/lib/x86_64-linux-gnu/libosmo-legacy-mgcp.so.0.0.0 found in none of the libraries
1 file changed