x86_64: Add asm/mtrr.h include for some builds

The earlier change to call the bp mtrr init from bugs.c broke
on some configurations due to missing includes.  Noticed
by "Avuton Olrich" <avuton@gmail.com>

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>