Fix flatpak build

python3-gobject is included in the Fedora flatpak runtime and is therefore
in /usr, but when ibus is built for flatpaks it installs to /app.

https://src.fedoraproject.org/rpms/ibus/pull-request/39

Resolves: RHEL-66126 configure option for Flatpak build

ibus.spec

@@ -380,6 +380,7 @@ autoreconf -f -i -v
 %else
     --enable-python-library \
 %endif
+    --with-python-overrides-dir=%{python3_sitearch}/gi/overrides \
     --enable-wayland \
     --enable-introspection \
     --enable-install-tests \