Fix compilation errors in usb/kshim

Remove old header from the include list and declare extern symbol
    for delay() function.

Approved by:           hselasky, cognet (mentor)
Differential revision: https://reviews.freebsd.org/D5012

sys/boot/kshim/bsd_global.h

@@ -55,7 +55,6 @@
 #include <dev/usb/usb_pf.h>
 #include <dev/usb/usb_request.h>
 #include <dev/usb/usb_util.h>
-#include <dev/usb/usb_compat_linux.h>
 #include <dev/usb/usbhid.h>
 #include <dev/usb/usb_ioctl.h>
 #include <dev/usb/usb_generic.h>

sys/boot/kshim/bsd_kernel.h

@@ -579,4 +579,7 @@ extern int (*ofw_bus_is_compatible_cb)(device_t dev, char *name);
 #define	strlcpy(d,s,n) snprintf((d),(n),"%s",(s))
 #endif
 
+/* Should be defined in user application since it is machine-dependent */
+extern int delay(unsigned int);
+
 #endif					/* _BSD_KERNEL_H_ */