ensure USB config numbers always start from 1

Depending on which features (and thus USB configurations) are included
in the firmware, we need to re-define the ordering of the configuration
numbers, as the Atmel USBD driver simply assumes that configurations are
numbered 1..N without any gaps in the sequence.
diff --git a/firmware/src_simtrace/usb.c b/firmware/src_simtrace/usb.c
index 6b57b1b..3d69780 100644
--- a/firmware/src_simtrace/usb.c
+++ b/firmware/src_simtrace/usb.c
@@ -32,6 +32,7 @@
  *----------------------------------------------------------------------------*/
 
 #include "board.h"
+#include "simtrace.h"
 #include "utils.h"
 
 #include <cciddriverdescriptors.h>