Add missing mkdirp prototype

For while now mkdirp has been built as part of libspl however
the protoype was never added to libgen.h.  This went unnoticed
until enabling the mount support which uses mkdirp().

lib/libspl/include/libgen.h

@@ -0,0 +1,34 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License, Version 1.0 only
+ * (the "License").  You may not use this file except in compliance
+ * with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright 2003 Sun Microsystems, Inc.  All rights reserved.
+ * Use is subject to license terms.
+ */
+
+#ifndef _LIBSPL_LIBGEN_H
+#define _LIBSPL_LIBGEN_H
+
+#include <sys/types.h>
+
+extern int mkdirp(const char *, mode_t);
+
+#endif /* _LIBSPL_LIBGEN_H */