Import lwIP contrib rep

... from http://git.savannah.gnu.org/cgit/lwip/lwip-contrib.git/ into contrib/ subdir, STABLE-2_1_0_RELEASE tag
lwIP contrib is now officially frozen
TODO: Fix build

contrib/examples/snmp/snmp_private_mib/private_mib.h

@@ -0,0 +1,26 @@
+/**
+ * @file
+ * Exports Private lwIP MIB 
+ */
+
+#ifndef LWIP_HDR_PRIVATE_MIB_H
+#define LWIP_HDR_PRIVATE_MIB_H
+
+#include "lwip/apps/snmp_opts.h"
+
+#include "lwip/apps/snmp_core.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* export MIB */
+extern const struct snmp_mib mib_private;
+
+void lwip_privmib_init(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif