Adding constant SOMAXCONN to vxworks

(backport <#3761>)
(cherry picked from commit 9d5b5a7)

src/vxworks/mod.rs

@@ -797,6 +797,7 @@ pub const S_IRWXO: ::c_int = 0x0007;
 
 // socket.h
 pub const SOL_SOCKET: ::c_int = 0xffff;
+pub const SOMAXCONN: ::c_int = 128;
 
 pub const SO_DEBUG: ::c_int = 0x0001;
 pub const SO_REUSEADDR: ::c_int = 0x0004;