doc: set docs.rs targets

Previously https://docs.rs/winapi-util went to the x86_64-unknown-linux-gnu
build of this crate by default, which is empty. This way the same link will
show useful documentation.

We achieve this by only building docs for one Windows target. Since the
API doesn't vary with other Windows targets, this seems fine.

PR BurntSushi#3

Author: dtolnay

Cargo.toml

@@ -26,3 +26,6 @@ features = [
   "winerror",
   "winnt",
 ]
+
+[package.metadata.docs.rs]
+targets = ["x86_64-pc-windows-msvc"]