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 <[e-mail]>
Acked-by: Shuah Khan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

tools/usb/usbip/libsrc/usbip_host_common.c

···
 #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)
 {