lib: fix ax25.h include for musl

ax25.h isn't guaranteed to be avilable in netax25/*;
it's dependent on our choice of libc (it's not available
on musl at least) [0].

Let's use the version from linux-headers.

[0] https://sourceware.org/glibc/wiki/Synchronizing_Headers
Bug: https://bugs.gentoo.org/831102

Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

lib/ax25_ntop.c

@@ -2,7 +2,7 @@
 
 #include <errno.h>
 #include <sys/socket.h>
-#include <netax25/ax25.h>
+#include <linux/ax25.h>
 
 #include "utils.h"