Fix folly/test/stl_tests/StlVectorTest.cpp under gcc8 with -Wparentheses
Fix folly/test/stl_tests/StlVectorTest.cpp under gcc8 with -Wparentheses

Summary:
[Folly] Fix `folly/test/stl_tests/StlVectorTest.cpp` under gcc8 with `-Wparentheses`.

```
folly/test/stl_tests/StlVectorTest.cpp: In function 'void test_copyConstruction(std::nullptr_t, Vector&, int)':
folly/test/stl_tests/StlVectorTest.cpp:1780:11: error: unnecessary parentheses in declaration of 'ca2' [-Werror=parentheses]
```

Reviewed By: Gownta

Differential Revision: D13604947

fbshipit-source-id: 36c0b596e07a19d4b282266574d1f4b837d5af4a

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly