Add android::fs_mgr namespace for new Fstab code

Test: treehugger
Change-Id: I69fb2af43842d1028582f249e767caa84687baa2
diff --git a/libavb_user/avb_ops_user.cpp b/libavb_user/avb_ops_user.cpp
index 3f9b9b0..d7815f0 100644
--- a/libavb_user/avb_ops_user.cpp
+++ b/libavb_user/avb_ops_user.cpp
@@ -39,6 +39,11 @@
 
 #include <libavb_ab/libavb_ab.h>
 
+using android::fs_mgr::Fstab;
+using android::fs_mgr::GetEntryForMountPoint;
+using android::fs_mgr::ReadDefaultFstab;
+using android::fs_mgr::ReadFstabFromFile;
+
 /* Open the appropriate fstab file and fallback to /fstab.device if
  * that's what's being used.
  */