Add 2021.3 feature

(We should add doing this as a SOP for ostree releases)

rust-bindings/rust/sys/Cargo.toml

@@ -55,6 +55,7 @@ v2020_7 = ["v2020_4"]
 v2020_8 = ["v2020_7"]
 v2021_1 = ["v2020_8"]
 v2021_2 = ["v2021_1"]
+v2021_3 = ["v2021_2"]
 
 [lib]
 name = "ostree_sys"
@@ -196,3 +197,6 @@ version = "2021.1"
 
 [package.metadata.system-deps.ostree_1.v2021_2]
 version = "2021.2"
+
+[package.metadata.system-deps.ostree_1.v2021_3]
+version = "2021.3"