In the lv_ex_conf.h file you need to set LV_EX_MOUSEWHEEL to 1 like so for the mousewheel function to be defined, but I think you will also need to have SDL2 installed.
#define LV_EX_PRINTF 1 /*Enable printf-ing data in demoes and examples*/
#define LV_EX_KEYBOARD 1 /*Add PC keyboard support to some examples (`lv_drivers` repository is required)*/
#define LV_EX_MOUSEWHEEL 1 /*Add 'encoder' (mouse wheel) support to some examples (`lv_drivers` repository is required)*/
Then define the symbols USE_MOUSE, USE_MOUSEWHEEL and USE_KEYBOARD.
I have installed SDL2 and now it compiles fine. But when I execute the program, I cannot move the focus with the mouse or keyboard. I was expecting that the mousewheel changed focus. I have set a breakpoint in the mousewheel_handler function, and the SDL_MOUSEWHEEL event does not trigger whatever I do… any clues?