gdbstub: Fix target_xml initialization

gdbstub: Fix target_xml initialization

target_xml is no longer a fixed-length array but a pointer to a
variable-length memory.

Fixes: 56e534bd11 ("gdbstub: refactor get_feature_xml")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230912224107.29669-2-akihiko.odaki@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231009164104.369749-9-alex.bennee@linaro.org>