serial-uclass: fix build warning

if CONFIG_DM_STDIO is defined but SERIAL_PRESENT not,
gcc drops warnings for serial_stub_* functions
that they are defined but not used.

Fix it.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/serial-uclass.c b/drivers/serial/serial-uclass.c
index 96a1cb6..362cedd 100644
--- a/drivers/serial/serial-uclass.c
+++ b/drivers/serial/serial-uclass.c
@@ -357,7 +357,6 @@
 {
 	_serial_putc(sdev->priv, ch);
 }
-#endif
 
 static void serial_stub_puts(struct stdio_dev *sdev, const char *str)
 {
@@ -374,6 +373,7 @@
 	return _serial_tstc(sdev->priv);
 }
 #endif
+#endif
 
 /**
  * on_baudrate() - Update the actual baudrate when the env var changes