headers_check fix: x86, setup.h

fix the following 'make headers_check' warning:

  usr/include/asm/setup.h:16: extern's make no sense in userspace
  usr/include/asm/setup.h:17: extern's make no sense in userspace
  usr/include/asm/setup.h:23: extern's make no sense in userspace
  usr/include/asm/setup.h:24: extern's make no sense in userspace
  usr/include/asm/setup.h:51: extern's make no sense in userspace
  usr/include/asm/setup.h:52: extern's make no sense in userspace

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