polkit: try harder to make polkit optional on non-Linux

We want the viewer to be usable on Mac/Windows/FreeBSD, even if our
profiler tooling isn't (yet).

Another option could be to do a simplified viewer window for those
platforms, but I think that is more work to maintain than abstracting
some of the stuff better in libsysprof.

src/libsysprof/meson.build

@@ -59,6 +59,7 @@ libsysprof_private_sources = [
   'sysprof-kallsyms.c',
   'sysprof-line-reader.c',
   'sysprof-map-lookaside.c',
+  'sysprof-polkit.c',
   'sysprof-symbol-map.c',
   ipc_service_src,
   stackstash_sources,