Add missing FFI def. for zmq_errno.

src/error.nobj.lua

@@ -224,6 +224,10 @@ meta_object "ZErrors" {
 	},
 }
 
+ffi_cdef[[
+int zmq_errno (void);
+]]
+
 ffi_source "ffi_src" [[
 -- get ZErrors table to map errno to error name.
 local ZError_names = _M.ZErrors
@@ -233,9 +237,6 @@ local function get_zmq_strerror()
 end
 ]]
 
-ffi_source "ffi_src" [[
-]]
-
 c_source "extra_code" [[
 static char *zmq_ZErrors_key = "zmq_ZErrors_key";
 /*