Add android::fs_mgr namespace for new Fstab code

Should have been done a while ago, but better late than never.

Test: treehugger
Change-Id: I0ea6e8d459cd3f3b3ce2d00a7a6a9786d52c52dd
diff --git a/init/builtins.cpp b/init/builtins.cpp
index 169edbe..9bedf40 100644
--- a/init/builtins.cpp
+++ b/init/builtins.cpp
@@ -75,6 +75,8 @@
 using namespace std::literals::string_literals;
 
 using android::base::unique_fd;
+using android::fs_mgr::Fstab;
+using android::fs_mgr::ReadFstabFromFile;
 
 #define chmod DO_NOT_USE_CHMOD_USE_FCHMODAT_SYMLINK_NOFOLLOW