kernel: Fix building of k_thread_create

commit ec7ecf79 moved some code around
such that the total_size variable is used regardless of how
CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT is set.  So move the
decleration of total_size outside of the ifndef block so things build
properly.

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