lkdtm: include cacheflush.h

Some architectures need cacheflush.h explicitly included (mips) for
use of flush_icache_range():

    config: make ARCH=mips allmodconfig
    All error/warnings:
    >> ERROR: "flush_icache_range" undefined!

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>