BUILD: connection: do not declare register_mux_proto() inline

This one is referenced in initcalls by its pointer, it makes no sense
to declare it inline. At best it causes function duplication, at worst
it doesn't build on older compilers.
diff --git a/src/connection.c b/src/connection.c
index c156d93..7d41471 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -1565,6 +1565,12 @@
 	return 0;
 }
 
+/* registers proto mux list <list>. Modifies the list element! */
+void register_mux_proto(struct mux_proto_list *list)
+{
+	LIST_APPEND(&mux_proto_list.list, &list->list);
+}
+
 /* Lists the known proto mux on <out>. This function is used by "haproxy -vv"
  * and is suitable for early boot just after the "REGISTER" stage because it
  * doesn't depend on anything to be already allocated.