otrx: include endian.h to make __BYTE_ORDER work with musl

Without this the condition
if __BYTE_ORDER == __BIG_ENDIAN
was always true.

Thanks Szabolcs.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46105 3c298f89-4303-0410-b956-a3cf2f4a3e73