x86, ds: fix compiler warning

Size_t is defined differently on i386 and x86_64.
Change type to avoid compiler warning.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Cc: roland@redhat.com
Cc: eranian@googlemail.com
Cc: oleg@redhat.com
Cc: juan.villacis@intel.com
Cc: ak@linux.jf.intel.com
LKML-Reference: <20090403144557.523964000@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/ds_selftest.c

@@ -87,7 +87,7 @@ static int ds_selftest_bts_read(struct bts_tracer *tracer,
 	/* Now to the test itself. */
 	for (at = from; (void *)at < to; at += trace->ds.size) {
 		struct bts_struct bts;
-		size_t index;
+		unsigned long index;
 		int error;
 
 		if (((void *)at - trace->ds.begin) % trace->ds.size) {