runtime: Disable default link feature on metal crate

Since we're only using a few types from the `metal` crate, this helper
library has no reason or need to link against framework libraries (i.e.
we don't call global static functions on them).  Disable the `link`
feature on the `metal` crate, but note that for now this won't disable
it on the `core-graphics-types` crate until
gfx-rs/metal-rs#330 is merged.

runtime/Cargo.toml

@@ -20,4 +20,4 @@ targets = [
 ]
 
 [dependencies]
-metal = "0.29"
+metal = { version = "0.29", default-features = false }