[libFuzzer] Fix __sanitizer_print_memory_profile return type in ExtFunctions.def

Summary:
Looks like a typo, as that function actually returns void and is used
as such in libFuzzer code as well.

Reviewers: kcc, Dor1s

Reviewed By: Dor1s

Subscribers: delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D65160

llvm-svn: 366834