modules: lvgl: Update OSAL implementation

The LVGL OSAL also tries to set the name of threads. Adjust the function
signature accordingly. Also deal with the fact that the type
`lv_thread_prio_t` is not part of the private api and requires a
LV_USE_PRIVATE_API symbol to be enabled.

Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>