selftests/intel_pstate: Fix warning on loop index overflow

The build was showing the warning:
 aperf.c:60:27: warning: iteration 2147483647 invokes undefined behavior
 [-Waggressive-loop-optimizations]
  for (i=0; i<0x8fffffff; i++) {

This change sets i, cpu and fd to unsigned int as they should not need
to be signed.

Cc: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: NStafford Horne <shorne@gmail.com>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/intel_pstate/aperf.c

@@ -14,7 +14,7 @@ void usage(char *name) {
 }
 
 int main(int argc, char **argv) {
-	int i, cpu, fd;
+	unsigned int i, cpu, fd;
 	char msr_file_name[64];
 	long long tsc, old_tsc, new_tsc;
 	long long aperf, old_aperf, new_aperf;