net: C++ support for mdns_responder.h

mdns_responder.h did not include header guards for C++, causing
it to fail if used in a C++ file. This change adds the header
guards.

Tested using samples/cpp/hello_world, with an added call to
mdns_responder_set_ext_records().

Signed-off-by: Andrew Kontra <[email protected]>

Author: Andrew Kontra

include/zephyr/net/mdns_responder.h

@@ -17,6 +17,10 @@
 #include <stddef.h>
 #include <zephyr/net/dns_sd.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @brief Register continuous memory of @ref dns_sd_rec records.
  *
@@ -31,4 +35,8 @@
  */
 int mdns_responder_set_ext_records(const struct dns_sd_rec *records, size_t count);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* ZEPHYR_INCLUDE_NET_MDNS_RESPONDER_H_ */