staging: usbip: userspace: fix header installation bug

A bug that I created due to using simply expanding variables in the
makefiles. Although only unexpanded paths are at issue here, I decided
to use recursively expanding variables on all of the parameterized
values.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>