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

arch/x86/kernel/tboot.c

@@ -33,6 +33,7 @@
 #include <asm/bootparam.h>
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
+#include <asm/fixmap.h>
 #include <asm/setup.h>
 #include <asm/tboot.h>
 #include <asm/e820.h>