Fix possible undefined reference when linking libusb from externals

When on Linux and using libusb from Externals, and with libudev
available, an "undefined reference" error message appears at link time.
This is due to a wrong define being set in CMake for libusb with
libudev. This causes the code for netlink being used instead of the
correct code for libudev support.

Fix the issue by setting the correct define so the correct libusb code
is used.

Externals/libusb/CMakeLists.txt

@@ -48,7 +48,7 @@ elseif(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
         target_link_libraries(usb PRIVATE "${LIBUDEV_LIBRARIES}")
         target_include_directories(usb PRIVATE "${LIBUDEV_INCLUDE_DIR}")
         set(HAVE_LIBUDEV TRUE)
-        set(USE_LIBUDEV TRUE)
+        set(USE_UDEV TRUE)
     else()
         target_sources(usb PRIVATE libusb/os/linux_netlink.c)
     endif()