serial: usb: Correct the usbtty_...() prototypes

The function declarations in serial.h are not in sync with what is
currently used in usbtty. Fix this by updating the header and including
it, to help catch future such problems.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/serial/usbtty.c b/drivers/serial/usbtty.c
index 76d9c8a..f1c1a26 100644
--- a/drivers/serial/usbtty.c
+++ b/drivers/serial/usbtty.c
@@ -11,6 +11,7 @@
 #include <config.h>
 #include <circbuf.h>
 #include <env.h>
+#include <serial.h>
 #include <stdio_dev.h>
 #include <asm/unaligned.h>
 #include "usbtty.h"