swupdategui-rs: bump to de64c91
Pick up the fix for grid navigation: build.rs did not detect
lv_gridnav_add() in the LVGL bindings when rustfmt is not available,
as in a Yocto build. The GUI was built without grid navigation and the
keypad could not move the focus.
Signed-off-by: Stefano Babic <stefano.babic@swupdate.org>