Add ucred to musl

Author: azdlowry

src/unix/notbsd/linux/musl/mod.rs

@@ -72,6 +72,12 @@ s! {
         pub mem_unit: ::c_uint,
         pub __reserved: [::c_char; 256],
     }
+    
+    pub struct ucred {
+        pub pid: ::pid_t,
+        pub uid: ::uid_t,
+        pub gid: ::gid_t,
+    }
 }
 
 pub const SFD_CLOEXEC: ::c_int = 0x080000;