staging: usbip: Fix build with --with-tcp-wrappers

With --with-tcp-wrappers=no specified, the build system reset LIBS to
the empty string and thus fails to link against libsysfs.

Signed-off-by: Dominik Paulus <dominik.paulus@fau.de>
Signed-off-by: Tobias Polzer <tobias.polzer@fau.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>