zbus: add MULTITHREADING dependency

Zbus uses mutexes internally that are available
only when MULTITHREADING is enabled so add
it to fix the following error:

/opt/zephyr-sdk-0.16.3/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/
12.2.0/../../../../arm-zephyr-eabi/bin/ld.bfd: zephyr/subsys/
zbus/libsubsys__zbus.a(zbus.c.obj): in function `k_mutex_init':
/builds/zephyr/mcuboot/zephyr/include/generated/syscalls/kernel.h:969:
undefined reference to `z_impl_k_mutex_init'

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>