samples: modules: lvgl: demos: increase native_sim mempool size

Increase native_sim_64 LVGL MEM_POOL_SIZE to accomodate flex_layout demo

Signed-off-by: Abderrahmane JARMOUNI <git@jarmouni.me>
diff --git a/samples/modules/lvgl/demos/boards/native_sim_native_64.conf b/samples/modules/lvgl/demos/boards/native_sim_native_64.conf
index c7a2f1f..3622031 100644
--- a/samples/modules/lvgl/demos/boards/native_sim_native_64.conf
+++ b/samples/modules/lvgl/demos/boards/native_sim_native_64.conf
@@ -2,4 +2,4 @@
 # SPDX-License-Identifier: Apache-2.0
 
 CONFIG_LV_COLOR_DEPTH_32=y
-CONFIG_LV_Z_MEM_POOL_SIZE=65536
+CONFIG_LV_Z_MEM_POOL_SIZE=75264