Include IMGDIFF operation in minor version 4 or up.

Try imgdiff if both source and target contains gzip.

Test: unit test & generated a payload with boot.img
Bug: 26675118
Change-Id: I2861d9b953ffbdec44fdfb42cff5687698ea205e
diff --git a/payload_consumer/payload_constants.h b/payload_consumer/payload_constants.h
index c3cd363..76d740f 100644
--- a/payload_consumer/payload_constants.h
+++ b/payload_consumer/payload_constants.h
@@ -43,6 +43,9 @@
 // The minor version that allows per-operation source hash.
 extern const uint32_t kOpSrcHashMinorPayloadVersion;
 
+// The minor version that allows IMGDIFF operation.
+extern const uint32_t kImgdiffMinorPayloadVersion;
+
 
 // The kernel and rootfs partition names used by the BootControlInterface when
 // handling update payloads with a major version 1. The names of the updated