On FreeBSD, add dependency on epoll-shim library.

FreeBSD doesn't provide timerfd and eventfd functions. These are implemented in
3rd party library called epoll-shim. Link targets requiring these functions to
this library.

spa/plugins/test/meson.build

@@ -3,6 +3,6 @@ test_sources = ['fakesrc.c', 'fakesink.c', 'plugin.c']
 testlib = shared_library('spa-test',
                           test_sources,
                           include_directories : [ spa_inc],
-                          dependencies : pthread_lib,
+                          dependencies : [pthread_lib, epoll_shim_dep],
                           install : true,
                           install_dir : '@0@/spa/test'.format(get_option('libdir')))