Merge pull request #201 from volcano0dr/urts

remove #![feature(ptr_offset_from)] in sgx_urts. Thanks to @volcano0dr !
diff --git a/sgx_libc/src/linux/x86_64/mod.rs b/sgx_libc/src/linux/x86_64/mod.rs
index 5e995b1..2fe633f 100644
--- a/sgx_libc/src/linux/x86_64/mod.rs
+++ b/sgx_libc/src/linux/x86_64/mod.rs
@@ -136,6 +136,7 @@
 pub type ino64_t = u64;
 pub type nfds_t = c_ulong;
 pub type pthread_t = c_ulong;
+pub type pid_t = i32;
 
 #[derive(Copy, Clone, Debug)]
 pub enum DIR {}
@@ -400,6 +401,12 @@
         pub pw_dir: *mut c_char,
         pub pw_shell: *mut c_char,
     }
+
+    pub struct ucred {
+        pub pid: pid_t,
+        pub uid: uid_t,
+        pub gid: gid_t,
+    }
 }
 
 pub const AT_FDCWD: c_int = -100;