make the dependencies platform specific

Cargo.toml

@@ -9,6 +9,8 @@ description = "a library for using child processes from multiple threads"
 keywords = ["command", "process", "child", "subprocess"]
 categories = ["os"]
 
-[dependencies]
+[target.'cfg(not(windows))'.dependencies]
 libc = "0.2.42"
+
+[target.'cfg(windows)'.dependencies]
 winapi = { version = "0.3.5", features = ["synchapi", "winbase", "winerror", "winnt"] }