gspell: enable vala for cross compilation

it's already enabled for native builds because of how `configure`
detects support, this enables it for cross builds too.
vala support is critical for downstream consumers like `gnome.geary`,
which fail if their dependencies don't provide .vapi files.

pkgs/development/libraries/gspell/default.nix

···
   buildInputs = [
     gtk3
     icu
+    vala # for share/vala/Makefile.vapigen (PKG_CONFIG_VAPIGEN_VAPIGEN)
   ];
 
   propagatedBuildInputs = [
···
   configureFlags = [
     "GLIB_COMPILE_RESOURCES=${lib.getDev buildPackages.glib}/bin/glib-compile-resources"
     "GLIB_MKENUMS=${lib.getDev buildPackages.glib}/bin/glib-mkenums"
+    "PKG_CONFIG_VAPIGEN_VAPIGEN=${lib.getBin buildPackages.vala}/bin/vapigen"
+    "--enable-introspection=yes"
+    "--enable-vala=yes"
   ];
 
   passthru = {