samples: lz4: reduce malloc arena size

PR zephyrproject-rtos#63332 introduced a change to the LZ4 sample that set the malloc
arena to an unnecessarily large size.
After testing on native_posix, qemu_m3, and esp32s3_devkitm_appcpu, it
would appear 24K is a much more reasonable (and sufficient) size.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

samples/compression/lz4/prj.conf

@@ -1,4 +1,4 @@
 CONFIG_LZ4=y
 CONFIG_REQUIRES_FULL_LIBC=y
 CONFIG_HEAP_MEM_POOL_SIZE=16384
-CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=65536
+CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=24576