Fix build of generated C++ code when using transitions on int properties with esp-idf

[tronical]

The energy monitor declares a transition on an animated int property, for which Property::set_animated_binding_for_transition is called, which in turn calls slint_property_set_animated_binding_helper. The latter is overloaded for various property types, such as float, Color, or Brush, and then calls specialized functions from ffi, such as

slint_property_set_animated_binding_(int|float|etc.).

slint_property_set_animated_binding_int uses i32 in Rust, which cbindgen maps to int32_t, so the
slint_property_set_animated_binding_helper overload also uses int32_t.

Unfortunately, with esp-idf, int32_t is a distinct type from int, and the overload resolution fails.

As remedy, this change uses c_int instead of i32 in the Rust ffi, which maps to int.

This seems easier than changing Property to Property<int32_t> :-)

[ogoffart]

Is there going to be a test?
Would it be possible to run the test-driver-cpp on such platform?

[tronical]

Is there going to be a test?

Best I can think of right now is implicit - getting the energy monitor for the esp32p4. (it renders one frame, fwiw)

Would it be possible to run the test-driver-cpp on such platform?

Perhaps a compile-only test would be possible, but linking may require going all the way idf.py. Not sure it's worth it - in this case riscv32-esp-elf-g++ is gcc and this is a bit of an edge case.