[binutils-gdb] btrace: fix output of "set record btrace"

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT b85310e1ec0419c4e1ca091cdd48f7597ebbefd3 ***

Author: Markus Metzger <markus.t.metzger@intel.com>
Branch: master
Commit: b85310e1ec0419c4e1ca091cdd48f7597ebbefd3

btrace: fix output of "set record btrace"

Instead of giving a message that "set record btrace" needs a sub-command,
GDB crashed.  Fix it.  A regression test comes with the next patch.

gdb/
	* record-btrace.c (cmd_set_record_btrace): Print sub-commands.