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_generator/payload_generation_config.h b/payload_generator/payload_generation_config.h
index 21bb89b..92ebbe5 100644
--- a/payload_generator/payload_generation_config.h
+++ b/payload_generator/payload_generation_config.h
@@ -125,6 +125,9 @@
   // Wheter the requested payload is a delta payload.
   bool is_delta = false;
 
+  // Wheter the IMGDIFF operation is allowed.
+  bool imgdiff_allowed = false;
+
   // The major_version of the requested payload.
   uint64_t major_version;