modules: lvgl: include lv_arclabel.c in Zephyr build

Add the lv_arclabel.c source file to modules/lvgl/CMakeLists.txt
so that the Arclabel widget is compiled and available in Zephyr projects.

Previously, the arclabel widget was not included, making it unusable.

Signed-off-by: Yehuda Eisenberg <yehuda@yehudae.net>