Fix a mistake in r217762

Summary:
The extra macro definition needs to go into COMPILER_RT_GTEST_CFLAGS
in order to be used for gtests on MSVC2012.

Test Plan: This is part of the fixes necessary for the ASAN tests to pass with MSVC2012.

Reviewers: timurrrr

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5493

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@218464 91177308-0d34-0410-b5e6-96231b3b80d8