kernel-netlink: Define SOL_NETLINK for old versions of socket.h

While the kernel defines it since 2005 (2.6.14), some older versions of
socket.h shipped with C libraries might not.  In particular, glibc only
added it with 2.24 in 2016.

Closes strongswan/strongswan#1503

src/libcharon/plugins/kernel_netlink/kernel_netlink_shared.c

@@ -52,6 +52,11 @@
 #include <collections/array.h>
 #include <collections/hashtable.h>
 
+/* some older versions of socket.h don't define this yet */
+#ifndef SOL_NETLINK
+#define SOL_NETLINK 270
+#endif
+
 typedef struct private_netlink_socket_t private_netlink_socket_t;
 
 /**