Include missing REPLACE_XZ switch case.

The operation was included in the .proto file but not in this switch
case.

Bug: 23179128
Test: emerge-link update_engine

Change-Id: I99887a1dbcf6237d268bcd0c117d3766f113df64
diff --git a/payload_generator/annotated_operation.cc b/payload_generator/annotated_operation.cc
index 3f28c8d..3fbea2c 100644
--- a/payload_generator/annotated_operation.cc
+++ b/payload_generator/annotated_operation.cc
@@ -64,6 +64,8 @@
       return "ZERO";
     case InstallOperation::DISCARD:
       return "DISCARD";
+    case InstallOperation::REPLACE_XZ:
+      return "REPLACE_XZ";
   }
   return "UNK";
 }