x86/intel_rdt: Add a missing #include

... to fix this build warning:

  In file included from arch/x86/kernel/cpu/intel_rdt.c:33:0:
  ./arch/x86/include/asm/intel_rdt.h:56:25: warning: ‘struct kernfs_open_file’ declared inside parameter list will not be visible outside of this definition or declaration
    int (*seq_show)(struct kernfs_open_file *of,
                           ^~~~~~~~~~~~~~~~
  ...
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <h.peter.anvin@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nilay Vaish <nilayvaish@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi V Shankar <ravi.v.shankar@intel.com>
Cc: Sai Prakhya <sai.praneeth.prakhya@intel.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Vikas Shivappa <vikas.shivappa@linux.intel.com>
Link: http://lkml.kernel.org/r/20161102165117.23545-1-bp@alien8.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/intel_rdt.h

@@ -3,6 +3,7 @@
 
 #ifdef CONFIG_INTEL_RDT_A
 
+#include <linux/kernfs.h>
 #include <linux/jump_label.h>
 
 #include <asm/intel_rdt_common.h>