oku: use system oniguruma and fix build with gcc15

pkgs/by-name/ok/oku/package.nix

@@ -9,6 +9,7 @@
   gtk4,
   hicolor-icon-theme,
   libadwaita,
+  oniguruma,
   pango,
   webkitgtk_6_0,
   nix-update-script,
@@ -45,10 +46,14 @@ rustPlatform.buildRustPackage (finalAttrs: {
     gtk4
     hicolor-icon-theme
     libadwaita
+    oniguruma
     pango
     webkitgtk_6_0
   ];
 
+  # use system oniguruma since the bundled one fails to build with gcc15
+  env.RUSTONIG_SYSTEM_LIBONIG = 1;
+
   # the program expects icons to be installed but the
   # program does not install them itself
   postInstall = ''