Add custom allocator feature

[AlixANNERAUD]

This feature is necessary when LV_MEM_CUSTOM=1 because lv_deinit() is not defined in this scenario :

error[E0425]: cannot find function `lv_deinit` in crate `lvgl_sys`
    --> /home/alix_anneraud/.cargo/git/checkouts/lv_binding_rust-d86feb7597e107b7/9829aef/lvgl/src/lib.rs:103:23
     |
103  |             lvgl_sys::lv_deinit();
     |                       ^^^^^^^^^ help: a function with a similar name exists: `lv_init`
     |
    ::: /home/alix_anneraud/Git/Personnel/Xila/New-Code/target/debug/build/lvgl-sys-927f327e46b48320/out/bindings.rs:7701:5
     |
7701 |     pub fn lv_init();
     |     ---------------- similarly named function `lv_init` defined here

For more information about this error, try `rustc --explain E0425`.
error: could not compile `lvgl` (lib) due to 1 previous error