Update snapcraft dependencies.

Author: michaelrsweet

snap/snapcraft.yaml

@@ -32,5 +32,5 @@ parts:
         plugin: autotools
         autotools-configure-parameters: [--prefix=/usr]
         source: .
-        build-packages: [build-essential, pkg-config, libfltk1.3-dev, libgnutls28-dev, libjpeg-turbo8-dev, libpng-dev, zlib1g-dev]
+        build-packages: [build-essential, pkg-config, libfltk1.3-dev, libcups2-dev, libjpeg-turbo8-dev, libpng-dev, zlib1g-dev]
         stage-packages: [libfltk-images1.3, libfltk1.3, libfontconfig1, libfreetype6, libjpeg-turbo8, libpng16-16, libx11-6, libxau6, libxcb1, libxcursor1, libxdmcp6, libxext6, libxfixes3, libxft2, libxinerama1, libxrender1]