[test] [runtime] Do not include alloca.h on NetBSD

On NetBSD, alloca() is in stdlib.h and there is no alloca.h.  Adjust
the includes appopriately.

Differential Revision: https://reviews.llvm.org/D55487

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@348856 91177308-0d34-0410-b5e6-96231b3b80d8