[vala] linux: add dup3

[Dr. Michael Lauer <mlauer src gnome org>]

commit 7e35732d613bcee4d58a2f50e304e1cf2b448190
Author: Dr. Michael Lauer <mickey vanille-media de>
Date:   Wed Feb 14 11:15:46 2018 +0100

    linux: add dup3
    
    https://bugzilla.gnome.org/show_bug.cgi?id=729803

 vapi/linux.vapi |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/vapi/linux.vapi b/vapi/linux.vapi
index 3c4c738..f65a36e 100644
--- a/vapi/linux.vapi
+++ b/vapi/linux.vapi
@@ -1060,6 +1060,8 @@ namespace Linux {
     }
 
     [CCode (cheader_filename = "unistd.h")]
+    public int dup3 (int oldfd, int newfd, int flags);
+    [CCode (cheader_filename = "unistd.h")]
     public int sethostname (string name, size_t len);
 
     // additional sysconf(3) constants