subsys/img_mgmt: Add zephyr/toolchain.h for STRINGIFY [v2]

Somehow two files in subsys/mgmt/mcumgr/lib/cmd/img_mgmt/src ended up
using STRINGIFY but nothing in their include path ended up pulling in
zephyr/toolchain/common.h. Include that via zephyr/toolchain.h.

v2:
	Use non-internal zephyr/toolchain.h header

Signed-off-by: Keith Packard <keithp@keithp.com>