qscintilla: add qtmacextras to propagatedBuildInputs on darwin

[ryan4729]

Motivation for this change

I found that qgis failed to build on macOS because python3Packages.qscintilla-qt5 was failing. The problem was that qtmacextras needs to be in the buildInputs of python3Packages.qscintilla-qt5. This change adds qtmacextras to propagatedBuildInputs of qscintilla. I wasn't sure if this is the right change, or if I should add qtmacextras directly to python3Packages.qscintilla-qt5.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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