libsysprof-profile: tail journald and append logs to capture

It can be handy to get system information from journald to correlate with
what is going on in applications. This simple journald tail GSource will
dispatch to our callback which can append the logs to the capture.

This uses a custom callback rather than the GSourceFunc because that seems
a bit annoying to use with recent GCC function equivalence checks.

config.h.meson

@@ -16,6 +16,8 @@
 
 #mesondefine HAVE_EXECINFO_H
 
+#mesondefine HAVE_LIBSYSTEMD
+
 #mesondefine HAVE_PERF_CLOCKID
 
 #mesondefine HAVE_POLKIT