flatpak: Pin gspell to 1.8.1

gspell currently has an issue that causes a build
failure. This commit is necessary to fix CI and development
builds until gspell is fixed.

flatpak/org.gnome.Fractal.json

@@ -75,8 +75,9 @@
             "name": "gspell",
             "sources" : [
                 {
-                    "type" : "git",
-                    "url" : "https://gitlab.gnome.org/GNOME/gspell.git"
+                    "type" : "archive",
+                    "url" : "https://download.gnome.org/sources/gspell/1.8/gspell-1.8.1.tar.xz",
+                    "sha256" : "819a1d23c7603000e73f5e738bdd284342e0cd345fb0c7650999c31ec741bbe5"
                 }
             ]
         },