encode.h: fix one-letter pre-processor symbol to avoid conflicts
encode.h: fix one-letter pre-processor symbol to avoid conflicts

It was suggested to pick a more specific name for the
pre-processor symbol E in encode.h to prevent name clashes,
see https://github.com/obgm/libcoap/issues/46.

gebart/libcoap,authmillenon/libcoap,gebart/libcoap,authmillenon/libcoap,authmillenon/libcoap,gebart/libcoap