kernel: msq_q: Fix compile warning
kernel: msq_q: Fix compile warning

Fix a compile warning if we build using int types defined to match the
compiler.  We get the following warnings:

kernel/msg_q.c: In function ‘_impl_k_msgq_alloc_init’:
kernel/msg_q.c:75:9: warning: passing argument 3 of ‘__builtin_umul_overflow’ from incompatible pointer type [-Wincompatible-pointer-types]
         (u32_t *)&total_size)) {
         ^
kernel/msg_q.c:75:9: note: expected ‘unsigned int *’ but argument is of type ‘u32_t * {aka long unsigned int *}’

__builtin_umul_overflow expects to be passed unsigned int for all its
arguments, so cast to that instead of u32_t.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,punitvara/zephyr,punitvara/zephyr,ldts/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,explora26/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr