x86, bts: exclude ds.c from build when disabled

Impact: cleanup

Move the CONFIG guard from the .c file into the makefile.

Reported-by: Andi Kleen <andi-suse@firstfloor.org>
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>