Better workaround for the `feature(rustc_private)]` issue

Now you don't need a janky `Xargo` fork anymore!

ctr-std/Cargo.toml

@@ -15,6 +15,7 @@ default-features = false
 
 [dependencies.ctru-sys]
 path = "../ctru-sys"
+features = ["stdbuild"]
 
 [features]
 citra = []

ctru-sys/Cargo.toml

@@ -7,3 +7,8 @@ license = "https://en.wikipedia.org/wiki/Zlib_License"
 [dependencies.libc]
 version = "0.2"
 default-features = false
+
+[features]
+# This is a dummy feature that only exists to work around a Xargo issue.
+# User code should not enable it.
+stdbuild = []