USB: linux/usb/Kbuild needs only "header-y", not "unifdef-y"

Given that none of the referenced header files test the proprocessor
conditional __KERNEL__, there's no point "unifdef"fing them.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>