Fix build on gcc 7
Fix build on gcc 7

Fixes #1738.

dtzWill/nix,rycee/nix,NixOS/nix,rycee/nix,shlevy/nix,ehmry/nix,shlevy/nix,dtzWill/nix,dtzWill/nix,ehmry/nix,NixOS/nix,layus/nix,layus/nix,NixOS/nix,shlevy/nix,pikajude/nix,peti/nix,peti/nix,vcunat/nix,dezgeg/nix,dtzWill/nix,shlevy/nix,vcunat/nix,pikajude/nix,NixOS/nix,layus/nix,layus/nix,zimbatm/nix,zimbatm/nix,dezgeg/nix,dezgeg/nix,ehmry/nix,dezgeg/nix,peti/nix,rycee/nix,dezgeg/nix,dtzWill/nix,NixOS/nix,shlevy/nix,vcunat/nix,layus/nix,ehmry/nix,ehmry/nix,vcunat/nix,pikajude/nix,peti/nix,peti/nix,zimbatm/nix,rycee/nix,ehmry/nix,zimbatm/nix,rycee/nix