compiling the net/sockets/echo sample in cpp

[sun681]

hi, I tried to modify the file name "socket_echo.c" to "socket_echo.cpp" to compile it as cpp (and the cmake accordingly) for the native_posix board.

it doesnt compile because a "conflict declaration" for the select function (something not working with socket_select.h file from the zephyr and the /usr/include/sys/select.h file together)

it happen only in cpp but not in c, anyone knows why?