gnome module: properly fallback to gtk-update-icon-cache

Commit a0cade8f introduced a typo and wrongly check for
gtk4-update-icon-cache twice.
If gtk4-update-icon-cache (gtk4) is not found, look for
gtk-update-icon-cache (gtk3) instead.

mesonbuild/modules/gnome.py

@@ -371,7 +371,7 @@ class GnomeModule(ExtensionModule):
             prog = state.find_program('gtk4-update-icon-cache', required=False)
             found = isinstance(prog, build.Executable) or prog.found()
             if not found:
-                prog = state.find_program('gtk4-update-icon-cache')
+                prog = state.find_program('gtk-update-icon-cache')
             icondir = os.path.join(datadir_abs, 'icons', 'hicolor')
             script = state.backend.get_executable_serialisation([prog, '-q', '-t', '-f', icondir])
             script.skip_if_destdir = True