Only include sys/boot.h if LINUX_BOOT_ABI is defined

Only include sys/boot.h if LINUX_BOOT_ABI is defined in
sys/arm/arm/machdep.c

Not doing this prevents kernels that do not define LINUX_BOOT_ABI from
being build with gcc (at least 4.2.1).

Reviewed by: mmel
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D8459