wic-image-minimal: only depend on syslinux on x86 targets

Following other examples, only depend on syslinux when targetting x86.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit c58fcc13)
Signed-off-by: Steve Sakoman <steve@sakoman.com>