NET: Correct potential missing goto label in case statement.

If neither CONFIG_CMD_PING or CONFIG_CMD_SNTP are defined but
CONFIG_CMD_DNS is, a compile-time error will occur due to the
absence of a goto label.

Signed-off-by: Gray Remlin <gryrmln@gmail.com>

net/net.c

@@ -1724,7 +1724,8 @@ static int net_check_prereq (proto_t protocol)
 			puts ("*** ERROR: `serverip' not set\n");
 			return (1);
 		}
-#if defined(CONFIG_CMD_PING) || defined(CONFIG_CMD_SNTP)
+#if defined(CONFIG_CMD_PING) || defined(CONFIG_CMD_SNTP) || \
+    defined(CONFIG_CMD_DNS)
     common:
 #endif