Some of the Zephyr include files need to be C++, not C based. The 'extern "C"'
wrapper in include/coap3/coap.h means that all #include are treated as being C.
Remove 'extern "C"' wrapper from include/coap3/coap.h and add wrapper to
include files after #include and before any function / structure definitions.
Add in a C++ build test for CI.
