Fix gdb.trace/trace-buffer-size.exp race.

Just the usual missing $gdb_prompt match:

 (gdb) tstatus
 No trace has been run on the target.
 Collected 0 trace frames.
 Trace buffer has 5242880 bytes of 5242880 bytes free (0% full).
 Trace will stop if GDB disconnects.
 Not looking at any trace frame.
 PASS: gdb.trace/trace-buffer-size.exp: get default buffer size
 (gdb) set trace-buffer-size 4
 (gdb) FAIL: gdb.trace/trace-buffer-size.exp: set trace buffer size 1

This fixes it.

gdb/testsuite/
2013-03-21  Pedro Alves  <palves@redhat.com>

	* gdb.trace/trace-buffer-size.exp (get default buffer size):
	Expect $gdb_prompt in gdb_test_multiple.
*** empty log message ***