You could do this a number a ways... probably the easiest way is to just add a border of 1 px set to your desired color (white in this case). Then on your last menu item, you could make it so the menu spans the entire width and therefore the final 1 px white border becomes part of the "gutter" assuming you have a white gutter.
The other way to do this is to calculate the width of your entire menu when your finished adding all the buttons you will add. Let's say for example your background is grey... so you have a div holder that is the same height as your menu will be and this holder spans the entire width and has a background color set to the grey you want. Then nested inside that, you will have another div which will hold your menu. So to get the desired effect, you first get the width of the entire menu system. Pretend that with the borders, your menu width comes to 704 px. To cover up the last border, you would just need to make the "nested" div which holds the menu 703 px and then set the overflow to "hidden".
So in the end, your code looks like this (please note, I am using arbitrary names for example purposes only):
<?php include_once ("menu"); ?>
Now for the styling:
width: 703 px;