boot: zephyr: add missing sys_clock_disable declaration

Add missing declaration of sys_clock_disable() to fix compiler warning.

Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>

boot/zephyr/main.c

@@ -48,6 +48,8 @@ struct arm_vector_table {
     uint32_t reset;
 };
 
+extern void sys_clock_disable(void);
+
 static void do_boot(struct boot_rsp *rsp)
 {
     struct arm_vector_table *vt;