mgmt: mcumgr: make mcumgr handler variable const

Ensure mcumgr handler variables are consts in order to make ld happy and
not emit the "LOAD segment with RWX permissions" warning

Signed-off-by: Dennis Grijalva <[email protected]>

Author: dennisgr102

include/zephyr/mgmt/mcumgr/mgmt/handlers.h

@@ -41,7 +41,7 @@ struct mcumgr_handler {
  * @param _init	Init function to be called (mcumgr_handler_init_t).
  */
 #define MCUMGR_HANDLER_DEFINE(name, _init)			\
-	STRUCT_SECTION_ITERABLE(mcumgr_handler, name) = {	\
+	const STRUCT_SECTION_ITERABLE(mcumgr_handler, name) = {	\
 		.init = _init,					\
 	}