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>

kernel/compiler_stack_protect.c

diff --git a/kernel/compiler_stack_protect.c b/kernel/compiler_stack_protect.c
index 2ee2d81..928fdc6 100644
--- a/kernel/compiler_stack_protect.c
+++ b/kernel/compiler_stack_protect.c
@@ -47,11 +47,11 @@
  * The canary value gets initialized in z_cstart().
  */
 #ifdef CONFIG_STACK_CANARIES_TLS
-__thread uintptr_t __stack_chk_guard;
+__thread volatile uintptr_t __stack_chk_guard;
 #elif CONFIG_USERSPACE
-K_APP_DMEM(z_libc_partition) uintptr_t __stack_chk_guard;
+K_APP_DMEM(z_libc_partition) volatile uintptr_t __stack_chk_guard;
 #else
-__noinit uintptr_t __stack_chk_guard;
+__noinit volatile uintptr_t __stack_chk_guard;
 #endif
 
 /**