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