headers_check fix: linux/elf.h

fix the following 'make headers_check' warnings:

  usr/include/linux/elf.h:379: extern's make no sense in userspace
  usr/include/linux/elf.h:387: extern's make no sense in userspace
  usr/include/linux/elf.h:401: extern's make no sense in userspace
  usr/include/linux/elf.h:402: extern's make no sense in userspace

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