libsysprof-capture: Cut off first 8 bytes of initial counter id

This ensures that the id won't be screwed by the 32 → 24 bit conversion
that happens implicitly when using SysprofCaptureCounter & family of
structs.

These structs are tightly packed and declare the id field as a 24-bit
unsigned, which doesn't necessarily match the counting that is done
via SysprofCollector.

This is by no means a definitive solution, but this gets things in a
more usable state until a better solution is implemented.

src/libsysprof-capture/sysprof-collector.c

@@ -424,7 +424,7 @@ sysprof_collector_get (void)
 #else
     self->tid = self->pid;
 #endif
-    self->next_counter_id = ((unsigned)self->pid << 16);
+    self->next_counter_id = (((unsigned)self->pid << 16) >> 8);
 
     pthread_mutex_lock (&control_fd_lock);