tests: kernel: userspace: explicitly declare function for the test

In the wake of moving the internal API header arm_core_mpu_dev.h
into arch/arm/cortex_m/mpu, we need to explicitly declare the
arm_core_mpu_disable() function in the userspace test. Note that
arm_core_mpu_disable() (as any other function in this internal
API) is not supposed to be called directly by kernel/application
functions; an exception is allowed in this test suite, so we are
able to test the MPU disabling functionality.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>