Set label "for" attributes for credentials dialog

Label tags should always indicate which input they are associated with.

vnc.html

@@ -305,11 +305,11 @@
             Credentials
         </div>
         <div id="noVNC_username_block">
-            <label>Username:</label>
+            <label for="noVNC_username_input">Username:</label>
             <input id="noVNC_username_input">
         </div>
         <div id="noVNC_password_block">
-            <label>Password:</label>
+            <label for="noVNC_password_input">Password:</label>
             <input id="noVNC_password_input" type="password">
         </div>
         <div>