[libFuzzer] Include FuzzedDataProvider.h in the test without "utils" subdir.

Summary:
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.

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: mgorny, delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D64440

llvm-svn: 365544