Fix btrace regression

Tom de Vries pointed out that my earlier patch:

    commit 873a185b
    Date:   Fri Dec 16 07:56:57 2022 -0700

	Don't use struct buffer in handle_qxfer_btrace

regressed gdb.btrace/reconnect.exp.  I didn't notice this because I
did not have libipt installed.

This patch fixes the bug.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30169


Tested-By: Bruno Larsen <blarsen@redhat.com>