headers_check fix: mn10300, setup.h

fix the following 'make headers_check' warnings:

  usr/include/asm-mn10300/setup.h:14: extern's make no sense in userspace
  usr/include/asm-mn10300/setup.h:15: extern's make no sense in userspace

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