weston-log-flight-rec: Add a global variable to access the ring buffer

With it add also a function which can be used in an assert()-like
situation to display the contents of the ring buffer. Within gdb
this call also be called if the program is loaded/still loaded into
memory.

The global variable will be used in a later patch to be accessed from a
python gdb script.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

include/libweston/weston-log.h

@@ -126,6 +126,9 @@ struct weston_log_subscription *
 weston_log_subscription_iterate(struct weston_log_scope *scope,
 				struct weston_log_subscription *sub_iter);
 
+void
+weston_log_flight_recorder_display_buffer(FILE *file);
+
 #ifdef  __cplusplus
 }
 #endif