Fix a warning with the latest nightly compiler
Fix a warning with the latest nightly compiler

The latest compiler warns about unnecessary parentheses in a position
that previous compilers ignored.

asomers/nix,carllerche/nix-rust,carllerche/nix-rust,geofft/nix-rust,nix-rust/nix,nix-rust/nix,geofft/nix-rust,carllerche/nix-rust,asomers/nix,geofft/nix-rust