Revert "str.h: Fix coap_make_str_const() for C++"
Revert "str.h: Fix coap_make_str_const() for C++"

This reverts commit 2309691770e267b64f7aca39cc8dd6cabf3f499a.

As the temporary object is cleaned up, the contents pointed to
by the coap_str_const_t object are destroyed.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap