headers_check fix: linux/nubus.h

fix the following 'make headers_check' warnings:

  usr/include/linux/nubus.h:297: extern's make no sense in userspace
  usr/include/linux/nubus.h:299: extern's make no sense in userspace
  usr/include/linux/nubus.h:303: extern's make no sense in userspace

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>