[benchmarks][lazydir] Tweak a header libfs header inclusion

An upcoming patch rearranges some forward declarations in libfs. This
exposes lazy_dir.cc needing to include an additional header to get the
definition of vdircookie_t.

Test: full system build with the fs change patched in
Change-Id: I94bb749bcfeab9d031be6ff7f899ace1ae1acf21