tracing: Add (optional) sysprof support

Currently only used to time the render loop.

To test you can run a nested phoc like:

WLR_BACKENDS=wayland sysprof-cli --force -- _build/src/phoc

and then look at the marks tab in sysprof capture.syscap

Inspired from what glib is doing.

Signed-off-by: Guido Günther agx@sigxcpu.org