Add missing constant S_ISVTX for vxworks

(backport <rust-lang#3768>)
[ Change to `c_int` rather than `mode_t`. We should make everything
  consistent at some point, just not a partial change here. - Trevor ]
(cherry picked from commit e9b1b9c)

src/vxworks/mod.rs

@@ -782,6 +782,7 @@ pub const S_IFSOCK: ::c_int = 0xc000;
 pub const S_ISUID: ::c_int = 0x0800;
 pub const S_ISGID: ::c_int = 0x0400;
 pub const S_ISTXT: ::c_int = 0x0200;
+pub const S_ISVTX: ::c_int = 0o1000;
 pub const S_IRUSR: ::c_int = 0x0100;
 pub const S_IWUSR: ::c_int = 0x0080;
 pub const S_IXUSR: ::c_int = 0x0040;