Add OpenCowReader interface method to dynamic partition control

Test: treehugger
Change-Id: I393fc36372f9e979da1a6c7e49c7782a7372a5b9
diff --git a/common/dynamic_partition_control_stub.cc b/common/dynamic_partition_control_stub.cc
index cfc9e2e..31975b3 100644
--- a/common/dynamic_partition_control_stub.cc
+++ b/common/dynamic_partition_control_stub.cc
@@ -96,6 +96,13 @@
   return nullptr;
 }
 
+FileDescriptorPtr DynamicPartitionControlStub::OpenCowReader(
+    const std::string& unsuffixed_partition_name,
+    const std::optional<std::string>&,
+    bool /*is_append */) {
+  return nullptr;
+}
+
 bool DynamicPartitionControlStub::MapAllPartitions() {
   return false;
 }