Define dummy '__init' instead of include "linux/init.h" if !__KERNEL__,
or can not pass checking. The related error (with allmodconfig under
hexagon):

    CHECK   include/asm (34 files)
  usr/include/asm/setup.h:22: included file 'linux/init.h' is not exported
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>