coap: Fix coap implentation with cxx

Closes https://github.com/espressif/esp-idf/issues/6804

components/coap/port/include/coap/coap_dtls.h

@@ -11,6 +11,11 @@
 #ifndef COAP_DTLS_H_
 #define COAP_DTLS_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "libcoap.h"
 #include "coap_time.h"
 #include "str.h"
 
@@ -627,5 +632,8 @@ void coap_dtls_set_log_level(int level);
  */
 int coap_dtls_get_log_level(void);
 
+#ifdef __cplusplus
+}
+#endif
 
 #endif /* COAP_DTLS_H */