Fix -Werror,-Wself-assign on big-endian systems

I was getting the following error when compiling cborencoder.c as part of
QtBase for FreeBSD MIPS64:
error: explicitly assigning value of variable of type 'uint64_t' (aka 'unsigned long') to itself [-Werror,-Wself-assign]
    v = cbor_htonll(v);

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>