Correctly implement a stub for ICowWriter::AddXorBlocks

Test: th
Change-Id: I70c174c4d44ef979993063256e3eededf18bd086
diff --git a/fs_mgr/libsnapshot/cow_writer.cpp b/fs_mgr/libsnapshot/cow_writer.cpp
index 51505bf..ef30e32 100644
--- a/fs_mgr/libsnapshot/cow_writer.cpp
+++ b/fs_mgr/libsnapshot/cow_writer.cpp
@@ -58,8 +58,8 @@
     return EmitRawBlocks(new_block_start, data, size);
 }
 
-bool AddXorBlocks(uint32_t /*new_block_start*/, const void* /*data*/, size_t /*size*/,
-                  uint32_t /*old_block*/, uint16_t /*offset*/) {
+bool ICowWriter::AddXorBlocks(uint32_t /*new_block_start*/, const void* /*data*/, size_t /*size*/,
+                              uint32_t /*old_block*/, uint16_t /*offset*/) {
     LOG(ERROR) << "AddXorBlocks not yet implemented";
     return false;
 }