idc: zephyr_idc: implement idc_send_msg for non-multicore platforms

Ensure idc_send_msg() is implemented for all platforms linking
zephyr_idc. For non-multicore, idc_send_msg() will return -ENOTSUP
error.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>

src/idc/zephyr_idc.c

@@ -54,6 +54,11 @@ void idc_init_thread(void)
 {
 }
 
+int idc_send_msg(struct idc_msg *msg, uint32_t mode)
+{
+	return -ENOTSUP;
+}
+
 #else
 
 K_P4WQ_ARRAY_DEFINE(q_zephyr_idc, CONFIG_CORE_COUNT, SOF_STACK_SIZE,