audio/gstreamer1-plugins-lv2: Add missing build dependencies

configure: * checking feature: lv2 *
configure: * for plug-ins: lv2 *
checking for LILV... no
checking for LILV... no
configure: *** These plugins will not be built: lv2

Package 'lv2', required by 'lilv-0', not found

http://beefy6.nyi.freebsd.org/data/120amd64-default/488500/logs/gstreamer1-plugins-lv2-1.14.4.log