sources/perf: Include the sample identifier in PERF_RECORD_SAMPLE.

If we want to start watching other perf events, we need a way to look
up what perf event we got when a PERF_RECORD_SAMPLE comes in.  The
sample identifier is what does that.

lib/sources/sp-perf-counter.h

@@ -81,6 +81,7 @@ typedef struct
 typedef struct
 {
   struct perf_event_header header;
+  guint64 identifier;
   guint64 ip;
   guint32 pid;
   guint32 tid;

lib/sources/sp-perf-source.c

@@ -291,6 +291,7 @@ sp_perf_source_start_pid (SpPerfSource  *self,
 
   attr.sample_type = PERF_SAMPLE_IP
                    | PERF_SAMPLE_TID
+                   | PERF_SAMPLE_IDENTIFIER
                    | PERF_SAMPLE_CALLCHAIN
                    | PERF_SAMPLE_TIME;
   attr.wakeup_events = N_WAKEUP_EVENTS;