perf buildid-list: Show running kernel build id fix

The --kernel option of perf buildid-list tool should show the running kernel buildid. The functionality has been lost during other changes
of the related code. The build_id__sprintf() function should return
length of the build-id string, but it was the length of the build-id raw data instead. Due to that, some return value checking caused that the final string was not printed out.

With this patch the build_id__sprintf() returns the correct value, so the --kernel option works again.

Before:

	# perf buildid-list --kernel
	#

After:

	# perf buildid-list --kernel
	972c1edab5bdc06cc224af45d510af662a3c6972
	#

---
 tools/perf/util/build-id.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 217b5a6..6a7e273 100644
@@ -91,7 +91,7 @@ int build_id__sprintf(const u8 *build_id, int len, char *bf)
 		bid += 2;
 	}
 
-	return raw - build_id;
+	return (bid - bf) + 1;
 }
 
 int sysfs__sprintf_build_id(const char *root_dir, char *sbuild_id)