[libFuzzer] Include FuzzedDataProvider.h in the test without "utils" subdir.
This way the test would better match the intended usage of the header,
plus it makes some additional testing (e.g. in CI) a bit easier to set up.
Reviewed By: morehouse
Subscribers: mgorny, delcypher, #sanitizers, llvm-commits
Tags: #llvm, #sanitizers
Differential Revision: https://reviews.llvm.org/D64440