merkaartor: fix run-time loading of Qt 5 platform
merkaartor: fix run-time loading of Qt 5 platform

Without this patch, running merkaartor fails with the following message:
“This application failed to start because it could not find or load the Qt
platform plugin "xcb"”.

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