fix staleness check in cmake/ctest (#2661)

diff --git a/compiler/cpp/test/compiler/Included.thrift b/compiler/cpp/test/compiler/Included.thrift
index 0406a65..fe039ed 100644
--- a/compiler/cpp/test/compiler/Included.thrift
+++ b/compiler/cpp/test/compiler/Included.thrift
@@ -19,6 +19,8 @@
  * under the License.
  */
 
+const string foo = "bar"
+
 struct a_struct {
   1: bool im_true,
   2: bool im_false,