baresip: enable the gtk feature
upstream's CMake feature detection expects the gtk3 package to supply
not only `cflags` but also `cflags_other`. i think the latter was
intended to be optional, so patch it out since our gtk doesn't supply it.
<https://github.com/baresip/baresip/blob/1501ad367dbdd0f91760e0def65e573bce29d0d8/cmake/FindGTK3.cmake#L14-L15>
authored by Colin and committed by Emery Hemingway 6d30a57f 0db1163a