tools: compiler.h: Fix build on FreeBSD

Commit 832472 "tools: socfpga: Add socfpga preloader signing
to mkimage" added tools/socfpga.c which relies on htole32,
le32toh and friends. While compiler.h includes these protypes
for linux from endian.h, it doesn't do so for FreeBSD. Hence
include <sys/endian.h> for FreeBSD.

Cc: Marek Vasut <marex@denx.de>
CC: Tom Rini <trini@ti.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>