[compiler-rt] Fix broken unittest using alloca on MSVC.

Summary:
The alloca header is not present on windows.

This test was committed recently:
  http://reviews.llvm.org/D21509
  http://reviews.llvm.org/rL273889

Reviewers: rnk

Subscribers: llvm-commits, wang0109, chrisha, kubabrecka

Differential Revision: http://reviews.llvm.org/D21864

llvm-svn: 274230