Add a guard around mpu_wrappers.c (#480)

* Add a guard around mpu_wrappers.c

This avoid linker errors when this file is accidently compiled in
projects using non-MPU ports.

Signed-off-by: Gaurav Aggarwal <[email protected]>

* Fix formatting check

Signed-off-by: Gaurav Aggarwal <[email protected]>

Author: aggarg