xen/arm: Move ARCH_PATCH_INSN_SIZE out of the header livepatch.h

xen/arm: Move ARCH_PATCH_INSN_SIZE out of the header livepatch.h

At the moment, ARCH_PATCH_INSN_SIZE is defined in the header
livepatch.h. However, this is also used in the alternative code.

Rather than including livepatch.h just for using the define, move it in
the header insn.h which seems more suitable.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Juergen Gross <jgross@suse.com>