coap_time.h -- files in our branch newer than in github/master

This looks strange. The file we have in our branch seems newer than the one the maintain in the github/master branch. And the changes done (i.e. the difference between the files) are introduced by Olaf B. (i.e. the libcoap dev team).

Leaving the file in our branch as it is.

@ban please check if this is ok as it is.