libadwaita: depend on sassc

[bayandin]

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Currently, libadwaita installs sassc binary, which creates a conflict with sassc formula:

$ ls $(brew --prefix libadwaita)/bin
adwaita-1-demo  sassc

From what I see, sassc is required only to build stylesheets for libadwaita: https://gitlab.gnome.org/GNOME/libadwaita/-/blob/cfed18f13b1801493532ea415cb1d6ed3a351c0d/src/stylesheet/meson.build#L5-9

So it should be enough to add sassc as a build-time dependency

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