x86, intel_txt: tboot.c needs <asm/fixmap.h>

arch/x86/kernel/tboot.c needs <asm/fixmap.h>.  In most configurations
that ends up getting implicitly included, but not in all, causing
build failures in some configurations.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Joseph Cihula <joseph.cihula@intel.com>
Cc: Shane Wang <shane.wang@intel.com>