Compiletest libc dependency can be unix-only

In main.rs libc is imported as:

    #[cfg(unix)]
    extern crate libc;

Author: dtolnay

src/tools/compiletest/Cargo.toml

@@ -10,6 +10,8 @@ filetime = "0.1"
 getopts = "0.2"
 log = "0.3"
 rustc-serialize = "0.3"
+
+[target.'cfg(unix)'.dependencies]
 libc = "0.2"
 
 [target.'cfg(windows)'.dependencies]