support for stats static userspace probes via systemtap

We currently only have probes for the logging sub-system.

This patch adds two tracepoints for tracing the performance
impact of statistics reporting: stat_start and stat_done.

They can be used to trace the amount of time a libosmocore-using
application spends in reporting/exporting statistics.  This includes
both the CPU time for encoding the statistics, as well as the system
calls for sending them.

Change-Id: I7208c45f6d051505dd2435305c67b4d26c0b1dd2
Related: OS#4311
Related: SYS#4877
diff --git a/src/probes.d b/src/probes.d
index c16eb1e..e4150f0 100644
--- a/src/probes.d
+++ b/src/probes.d
@@ -1,4 +1,6 @@
 provider libosmocore {
 	probe log_start();
 	probe log_done();
+	probe stats_start();
+	probe stats_done();
 };