added validation message to language file for required_with

Signed-off-by: Jason Walton <jwalton512@gmail.com>

application/language/en/validation.php

@@ -58,6 +58,7 @@ return array(
 	"not_in"         => "The selected :attribute is invalid.",
 	"numeric"        => "The :attribute must be a number.",
 	"required"       => "The :attribute field is required.",
+    "required_with"  => "The :attribute field is required with :field",
 	"same"           => "The :attribute and :other must match.",
 	"size"           => array(
 		"numeric" => "The :attribute must be :size.",
@@ -101,4 +102,4 @@ return array(
 
 	'attributes' => array(),
 
-);
+);