examples/Makefile.am: fix include path
examples/Makefile.am: fix include path

Example applications are built against the libcoap public API
and therefore prefix libcoap includes with "coap3)/".

The only exception is coap-rd which uses uthash.h from the internal
API. Therefore, "-Iinclude/coap3" is additionally passed as CPPFLAG
for coap-rd.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap