libcoap: fix big endian problems

libcoap: fix big endian problems

When this is build for a big endian Linux system WORDS_BIGENDIAN was
not be set. This patch sets it correctly so it will not generate broken
code on big endian systems.
This was tested on MIPS BE 32 Bit.

Change-Id: I59dd07d8020c553318e2aa43894a2185fe9b9286
Signed-off-by: Hauke Mehrtens <hauke.mehrtens@lantiq.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/1219
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Erich Keane <erich.keane@intel.com>