mxsboot: remove unused include

Commit 276d3ebb removed htole32() but missed
to remove the corresponding header. This is annoying, since BSD systems do not
have endian.h.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Michael Heimpold <mhei@heimpold.de>