core: Add runtime ostree_check_version()

[Previously](https://github.com/ostreedev/ostree/pull/728) we added compile-time
checking for versions, but there are use cases for runtime checking as well,
because in a number of API calls we use `GVariant` as an API extension
mechanism.

Closes: #735
Approved by: jlebon

src/libostree/ostree-core.h

@@ -399,4 +399,7 @@ gchar *  ostree_commit_get_parent            (GVariant  *commit_variant);
 _OSTREE_PUBLIC
 guint64  ostree_commit_get_timestamp         (GVariant  *commit_variant);
 
+_OSTREE_PUBLIC
+gboolean ostree_check_version (guint required_year, guint required_release);
+
 G_END_DECLS