Staging: usbip: Fix out-of-tree build for userspace

This fixes out-of-tree builds ($builddir != $srcdir). In such cases,
libusbip.la cannot be found because it is generated in the build
directory and not the source directory.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: matt mooney <mfm@muteddisk.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>