teensy3: usb_dev: Include <usb_serial.h>

Include <usb_serial.h> instead of duplicating the needed forward
declarations.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

teensy3/usb_dev.h

         return usb_rx_byte_count_data[endpoint];
 }
 
-#ifdef CDC_DATA_INTERFACE
-extern uint32_t usb_cdc_line_coding[2];
-extern volatile uint32_t usb_cdc_line_rtsdtr_millis;
-extern volatile uint32_t systick_millis_count;
-extern volatile uint8_t usb_cdc_line_rtsdtr;
-extern volatile uint8_t usb_cdc_transmit_flush_timer;
-extern void usb_serial_flush_callback(void);
-#endif
-
 #ifdef SEREMU_INTERFACE
 extern volatile uint8_t usb_seremu_transmit_flush_timer;
 extern void usb_seremu_flush_callback(void);
 }
 #endif
 
+#ifdef CDC_DATA_INTERFACE
+#include "usb_serial.h"
+#endif
+
 #else // F_CPU < 20000000
 
 #ifdef __cplusplus