net: zoap: add missing response code for zoap_header_get_code()

Three response codes are missed in zoap_header_get_code() which will
result in the response code returned from the function being set as
ZOAP_CODE_EMPTY. Check include/net/zoap.h for the missing code
definition.

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>