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: NJaswinder Singh Rajput <jaswinderrajput@gmail.com>