usbip: tools: fix build error for multiple definition

With GCC 10, building usbip triggers error for multiple definition
of 'udev_context', in:
- libsrc/vhci_driver.c:18 and
- libsrc/usbip_host_common.c:27.

Declare as extern the definition in libsrc/usbip_host_common.c.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20200618000844.1048309-1-borneo.antonio@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>