roken: declare IN_LOOPBACKNET if necessary

Not all platforms provide a definition of
IN_LOOPBACKNET.  Let roken define it if it is otherwise
undeclared.

Change-Id: Ibbac2fc4045671237fae23119746b37afb3b83fb

lib/roken/roken.h.in

@@ -105,6 +105,10 @@ typedef int rk_socket_t;
 
 #endif
 
+#ifndef IN_LOOPBACKNET
+#define IN_LOOPBACKNET 127
+#endif
+
 #ifdef _MSC_VER
 /* Declarations for Microsoft Visual C runtime on Windows */