[compiler-rt] FuzzedDataProvider: use C++ headers only instead of a C/C++ mix.

Reviewers: Dor1s

Reviewed By: Dor1s

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

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D66017

llvm-svn: 368448