logging: fix the open source build

Summary:
Add ConfigHelpers.cpp to the folly_test_support library.
This was accidentally broken in D7164831, but wasn't caught due to
infrastructure errors kicking off the continuous OSS builds.

Reviewed By: yfeldblum

Differential Revision: D7186076

fbshipit-source-id: f13bdd920bb289ae19fdb46e265123fe088b42c3

CMakeLists.txt

@@ -350,6 +350,8 @@ if (BUILD_TESTS)
     ${FOLLY_DIR}/test/SingletonTestStructs.cpp
     ${FOLLY_DIR}/test/SocketAddressTestHelper.cpp
     ${FOLLY_DIR}/test/SocketAddressTestHelper.h
+    ${FOLLY_DIR}/experimental/logging/test/ConfigHelpers.cpp
+    ${FOLLY_DIR}/experimental/logging/test/ConfigHelpers.h
     ${FOLLY_DIR}/experimental/logging/test/TestLogHandler.cpp
     ${FOLLY_DIR}/experimental/logging/test/TestLogHandler.h
     ${FOLLY_DIR}/futures/test/TestExecutor.cpp