samples: modules: lvgl: demos: Increase memory pool to fix crash

Due to the added demos, a memory pool of 49 KiB is no longer enough and
leads to crashes:

https://github.com/lvgl/lvgl/issues/7648



Increasing the memory pool to 57 KiB fixes the problem.

Signed-off-by: Detlev Zundel <dzu@member.fsf.org>