libsysprof: add muxer GSource

This allows copying events from a capture stream transparently into the
destination. No processing of the stream is performed, but that may change
in the future to accomidate JIT/Counter translations.

Internal only as support for upcoming live unwinding via external process.

src/libsysprof/meson.build

@@ -147,6 +147,7 @@ libsysprof_private_sources = [
   'sysprof-mount-device.c',
   'sysprof-mount-namespace.c',
   'sysprof-mount.c',
+  'sysprof-muxer-source.c',
   'sysprof-perf-event-stream.c',
   'sysprof-podman.c',
   'sysprof-process-info.c',