Not every platform requires VRPN_LIBUSB_INCLUDE_DIR to be set

So far its only Ubuntu as the newer versions moved things around

Change-Id: Ia0e531fb84eea25df1715ae0ece2799e4aab8bff
Merge topic 'vr_vrpn_usb_optional' into next

f17a662 Not every platform requires VRPN_LIBUSB_INCLUDE_DIR to be set