[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
diff --git a/bin/zircon_benchmarks/lazy_dir.cc b/bin/zircon_benchmarks/lazy_dir.cc
index 458491f..2c41703 100644
--- a/bin/zircon_benchmarks/lazy_dir.cc
+++ b/bin/zircon_benchmarks/lazy_dir.cc
@@ -8,6 +8,7 @@
 #include <fbl/ref_ptr.h>
 #include <fbl/string_printf.h>
 #include <fs/lazy-dir.h>
+#include <fs/vfs.h>
 #include <lib/fxl/strings/string_printf.h>
 #include <perftest/perftest.h>
 #include <zircon/syscalls.h>