usb: Make usb_time_sigatt variable static

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/hw/usb.c b/src/hw/usb.c
index 20731d1..4f9a852 100644
--- a/src/hw/usb.c
+++ b/src/hw/usb.c
@@ -456,7 +456,7 @@
     goto done;
 }
 
-u32 usb_time_sigatt;
+static u32 usb_time_sigatt;
 
 void
 usb_enumerate(struct usbhub_s *hub)