btrace: Export btrace_decode_error function.

Signed-off-by: Tim Wiederhake <tim.wiederhake@intel.com>

gdb/ChangeLog:

	* record-btrace.c (btrace_ui_out_decode_error): Move most of it ...
	* btrace.c (btrace_decode_error): ... here.  New function.
	* btrace.h (btrace_decode_error): New export.

Change-Id: I2b4b43a55dbfd9f526a540d2ad52a6708f31feba