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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/usb/usbip/libsrc/usbip_host_common.c

@@ -23,7 +23,7 @@
 #include "list.h"
 #include "sysfs_utils.h"
 
-struct udev *udev_context;
+extern struct udev *udev_context;
 
 static int32_t read_attr_usbip_status(struct usbip_usb_device *udev)
 {