hide the linkhack module in the docs

Author: thestinger

src/libcore/core.rc

@@ -56,6 +56,7 @@ Implicitly, all crates behave as if they included the following prologue:
 
 // On Linux, link to the runtime with -lrt.
 #[cfg(target_os = "linux")]
+#[doc(hidden)]
 pub mod linkhack {
     #[link_args="-lrustrt -lrt"]
     #[link_args = "-lpthread"]