kernel: Unified type of stack protection canary __stack_chk_guard.

Extern declaration of __stack_chk_guard added volatile to
the type while the declaration was non-volatile. This cause
type check errors with compilers that declares the
__stack_chk_guard variable in an internal pre-include
header file (IAR).

While I think the volatile keyword is unnecessary, I decided
on keep it and add it to the declaration in
kernel/compiler_stack_protect.c

Tested with IAR ICCARM and the Zephyr SDK GCC.

Signed-off-by: Lars-Ove Karlsson <lars-ove.karlsson@iar.com>