ll_map: use net/if.h to get prototype

Better to get prototype from system headers

lib/ll_map.c

@@ -18,13 +18,11 @@
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <string.h>
-#include <linux/if.h>
+#include <net/if.h>
 
 #include "libnetlink.h"
 #include "ll_map.h"
 
-extern unsigned int if_nametoindex (const char *);
-
 struct ll_cache
 {
 	struct ll_cache   *idx_next;