Placate unused variable warnings uncovered by improvements to clang's -Wunused-variable

llvm-svn: 315809
diff --git a/libcxx/test/std/utilities/utility/synopsis.pass.cpp b/libcxx/test/std/utilities/utility/synopsis.pass.cpp
index 5f5b4ee..009b65c 100644
--- a/libcxx/test/std/utilities/utility/synopsis.pass.cpp
+++ b/libcxx/test/std/utilities/utility/synopsis.pass.cpp
@@ -17,5 +17,6 @@
 int main()
 {
     std::initializer_list<int> x;
+    (void)x;
 }