mypaint: fix crash without hicolor-icon-theme
mypaint: fix crash without hicolor-icon-theme

Mypaint seems to check for a presence of this theme scaffold and crashes when not present.
Let’s add this since it weights almost nothing.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs