samples: net: mqtt_publisher: increase num of thread to allowed

according to the latest build log, we need increase the MAX_THREEED

/usr/bin/python3 scripts/build/gen_kobject_list.py \
--kernel twister-out/frdm_k64f/samples/net/mqtt_publisher/\
sample.net.mqtt_publisher.userspace/zephyr/zephyr_pre0.elf \
--gperf-output kobject_prebuilt_hash.gperf --include-subsystem-list \
twister-out/frdm_k64f/samples/net/mqtt_publisher/\
sample.net.mqtt_publisher.userspace/zephyr/misc/generated/struct_tags.json
Too many thread objects (17)
Increase CONFIG_MAX_THREAD_BYTES to 3

Signed-off-by: Hake Huang <hake.huang@oss.nxp.com>