dd: fix testsuite

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/dd/dd-count-bytes

@@ -1 +1,3 @@
+# FEATURE: CONFIG_FEATURE_DD_IBS_OBS
+
 test "$(echo I WANT | busybox dd count=3 iflag=count_bytes 2>/dev/null)" = "I W"