net/mediastreamer: Explicitly link with libthr after base r345349

This should unbreak the build of net/liblinphone.

ld: error: /usr/local/lib/libmediastreamer_base.so.10: undefined reference to pthread_setschedparam

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p501607_s347555/logs/errors/liblinphone-3.12.0_1.log