NetBSD compatibility nit in asan_test_utils.h

Summary:
Do not include <malloc.h> on NetBSD, as this header
serves on this OS backward compatibility with K&R alias
for <stdlib.h>.

Sponsored by <The NetBSD Foundation>

Reviewers: vitalybuka, kcc, joerg, filcab, fjricci

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36469

llvm-svn: 310391