netlink: workaround to build against libnl3 < 3.2.20

Commit 418a428 fixed building against libnl3 >= 3.2.20 but due to a bug
in libnl it also prevents building against libnl3 < 3.2.20.

See https://bugzilla.gnome.org/show_bug.cgi?id=692423

src/nm-netlink-monitor.c

@@ -38,6 +38,10 @@
 #include <linux/if.h>
 #include <linux/unistd.h>
 #include <unistd.h>
+#include <netlink/version.h>
+#if LIBNL_VER_NUM == LIBNL_VER (3, 2) && LIBNL_VER_MIC < 20
+#include <netlink/object-api.h>
+#endif
 #include <netlink/object.h>
 #include <netlink/route/addr.h>
 #include <netlink/route/rtnl.h>