[sw] Promulgate better use of standard headers.

Signed-off-by: Miguel Young de la Sota <mcyoung@google.com>
diff --git a/sw/device/lib/usb_simpleserial.h b/sw/device/lib/usb_simpleserial.h
index dbb994a..daad16b 100644
--- a/sw/device/lib/usb_simpleserial.h
+++ b/sw/device/lib/usb_simpleserial.h
@@ -5,6 +5,9 @@
 #ifndef __USB_SIMPLESERIAL_H__
 #define __USB_SIMPLESERIAL_H__
 
+#include <stddef.h>
+#include <stdint.h>
+
 #include "sw/device/lib/common.h"
 #include "usbdev.h"