Add function CleanupOldScratchFiles symmetrically

For symmetry, add function CleanupOldScratchFiles in conditional compilation blocks which missing it.

Test: monkey test for one day and one night

Signed-off-by: Jintao Zhu <zhujtcsieee@gmail.com>

Change-Id: Ie754427334c9a9bb7cfed70df45f439c60c9ab16
diff --git a/fs_mgr/fs_mgr_overlayfs.cpp b/fs_mgr/fs_mgr_overlayfs.cpp
index 1134f14..cb09383 100644
--- a/fs_mgr/fs_mgr_overlayfs.cpp
+++ b/fs_mgr/fs_mgr_overlayfs.cpp
@@ -116,6 +116,8 @@
 void MapScratchPartitionIfNeeded(Fstab*, const std::function<bool(const std::set<std::string>&)>&) {
 }
 
+void CleanupOldScratchFiles() {}
+
 void TeardownAllOverlayForMountPoint(const std::string&) {}
 
 }  // namespace fs_mgr