drop unused libc imports on L4Re

As a capability-based microkernel OS, L4Re only has incomplete support
for POSIX APIs, in particular it does not implement UIDs and GIDs.

library/std/src/sys/unix/process/process_unix.rs

@@ -12,12 +12,12 @@ use crate::sys::process::process_common::*;
 #[cfg(target_os = "vxworks")]
 use libc::RTP_ID as pid_t;
 
-#[cfg(not(target_os = "vxworks"))]
-use libc::{c_int, gid_t, pid_t, uid_t};
-#[cfg(not(target_os = "l4re"))]
-use libc::{gid_t, uid_t};
+#[cfg(target_os = "l4re")]
 use libc::{c_int, pid_t};
 
+#[cfg(not(any(target_os = "vxworks", target_os = "l4re")))]
+use libc::{c_int, gid_t, pid_t, uid_t};
+
 ////////////////////////////////////////////////////////////////////////////////
 // Command
 ////////////////////////////////////////////////////////////////////////////////