fix -Wvexing-parse

fix -Wvexing-parse

Summary:
```lang=cpp
folly/test/stl_tests/StlVectorTest.cpp:1975:11: error: parentheses were disambiguated as redundant parentheses around declaration of variable named 'm' [-Werror,-Wvexing-parse]
    Vector(m);
          ^~~
folly/test/stl_tests/StlVectorTest.cpp:1975:11: note: add a variable name to declare a 'Vector' initialized with 'm'
    Vector(m);
          ^
           varname
folly/test/stl_tests/StlVectorTest.cpp:1975:5: note: add enclosing parentheses to perform a function-style cast
    Vector(m);
    ^
    (        )
folly/test/stl_tests/StlVectorTest.cpp:1975:11: note: remove parentheses to silence this warning
    Vector(m);
          ^ ~

1 error generated.
```

Reviewed By: Gownta

Differential Revision: D5955613

fbshipit-source-id: 3015570ce111603b3561914d07b55365a221c686