usb:msc: include referenced usbd_device headers

In addition to stdint.h, MSC directly relies on the usbd_device type
definition existing.  Ensure it has included it's own dependencies.

Fixes: https://github.com/libopencm3/libopencm3/issues/1471
Signed-off-by: Karl Palsson <karlp@tweak.au>

include/libopencm3/usb/msc.h

@@ -41,6 +41,7 @@ LGPL License Terms @ref lgpl_license
 #define __MSC_H
 
 #include <stdint.h>
+#include <libopencm3/usb/usbd.h>
 
 typedef struct _usbd_mass_storage usbd_mass_storage;