Pretend the <fts.h> functions didn't appear until 21.

Because there was an accidental ABI breakage in 21, and we can't fix
the past. It seems more useful to refuse to build code that won't work
on any current or future device.

Bug: https://issuetracker.google.com/37051826
Test: builds
Change-Id: I4b5c30f899e2c7307642e2fb20e43a3ca14ae7fe