Add tracing probes to the isc_job unit

Add tracing probes to isc_job unit:

 * libisc:job_cb_before - before the job callback is called
 * libisc:job_cb_after - after the job callback is called

tests/isc/job_test.c

@@ -31,8 +31,6 @@
 #include <isc/result.h>
 #include <isc/util.h>
 
-#include "job.c"
-
 #include <tests/isc.h>
 
 static atomic_uint scheduled;