serial: make local functions static

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
diff --git a/common/stdio.c b/common/stdio.c
index 8232815..68c595d 100644
--- a/common/stdio.c
+++ b/common/stdio.c
@@ -39,39 +39,39 @@
 #endif
 
 #ifdef CONFIG_SYS_DEVICE_NULLDEV
-void nulldev_putc(struct stdio_dev *dev, const char c)
+static void nulldev_putc(struct stdio_dev *dev, const char c)
 {
 	/* nulldev is empty! */
 }
 
-void nulldev_puts(struct stdio_dev *dev, const char *s)
+static void nulldev_puts(struct stdio_dev *dev, const char *s)
 {
 	/* nulldev is empty! */
 }
 
-int nulldev_input(struct stdio_dev *dev)
+static int nulldev_input(struct stdio_dev *dev)
 {
 	/* nulldev is empty! */
 	return 0;
 }
 #endif
 
-void stdio_serial_putc(struct stdio_dev *dev, const char c)
+static void stdio_serial_putc(struct stdio_dev *dev, const char c)
 {
 	serial_putc(c);
 }
 
-void stdio_serial_puts(struct stdio_dev *dev, const char *s)
+static void stdio_serial_puts(struct stdio_dev *dev, const char *s)
 {
 	serial_puts(s);
 }
 
-int stdio_serial_getc(struct stdio_dev *dev)
+static int stdio_serial_getc(struct stdio_dev *dev)
 {
 	return serial_getc();
 }
 
-int stdio_serial_tstc(struct stdio_dev *dev)
+static int stdio_serial_tstc(struct stdio_dev *dev)
 {
 	return serial_tstc();
 }