[Kernel] Fix the signal compiling issue when using minilibc

components/libc/compilers/minilibc/signal.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2006-2018, RT-Thread Development Team
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Change Logs:
+ * Date           Author       Notes
+ * 2017-09-12     Bernard      The first version
+ */
+
+#ifndef SIGNAL_H__
+#define SIGNAL_H__
+
+#include <libc/libc_signal.h>
+
+#define SIG_DFL ((_sig_func_ptr)0)  /* Default action */
+#define SIG_IGN ((_sig_func_ptr)1)  /* Ignore action */
+#define SIG_ERR ((_sig_func_ptr)-1) /* Error return */
+
+#endif