samples: net: coap_client: Fix coverity issue

Ignore socket close() return value in this sample.

Fixes #18961

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>

samples/net/sockets/coap_client/src/coap-client.c

@@ -627,14 +627,14 @@ void main(void)
 	}
 
 	/* Close the socket */
-	close(sock);
+	(void)close(sock);
 
 	LOG_DBG("Done");
 
 	return;
 
 quit:
-	close(sock);
+	(void)close(sock);
 
 	LOG_ERR("quit");
 }