Add genl_socket_close() function

We've had genl_socket_open() but no corresponding genl_socket_close()
so far.  Let's fix that.
diff --git a/src/genl.c b/src/genl.c
index 311480c..f534221 100644
--- a/src/genl.c
+++ b/src/genl.c
@@ -106,6 +106,12 @@
 }
 EXPORT_SYMBOL(genl_socket_open);
 
+void genl_socket_close(struct mnl_socket *nl)
+{
+	mnl_socket_close(nl);
+}
+EXPORT_SYMBOL(genl_socket_close);
+
 int genl_socket_talk(struct mnl_socket *nl, struct nlmsghdr *nlh, uint32_t seq,
 		     int (*cb)(const struct nlmsghdr *nlh, void *data),
 		     void *data)