staging: usbip: userspace: allow `configure --with-tcp-wrappers`

When `--with-tcp-wrappers` is passed to `configure`, the previous code
always reset LIBS to $saved_LIBS, regardless of whether libwrap was
found or not.  The current code makes the `--with-tcp-wrappers` case
look more like the default case, and it only resets LIBS if libwrap
was not found.

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>