Cargo: don't let feature "static" pull xenstore-rs unless "xenstore" is set

The syntax used was equivalent to "dep:xenstore-rs/static", and a bug in
Rust 1.70 (fixed by rust-lang/cargo#12130 in 1.71)
was triggered.

Author: ydirson

Cargo.toml

@@ -39,7 +39,7 @@ sysctl = "0.5.0"
 [features]
 default = ["xenstore", "net_netlink"]
 xenstore = ["dep:xenstore-rs"]
-static = ["xenstore-rs/static"]
+static = ["xenstore-rs?/static"]
 net_netlink = ["dep:netlink-proto", "dep:netlink-packet-core", "dep:netlink-packet-route",
                "dep:rtnetlink"]
 net_pnet = ["dep:pnet_datalink", "dep:pnet_base", "dep:ipnetwork"]