Add ms-viewport to responsive.less

According to the October 15th update on this page (http://mattstow.com/responsive-design-in-ie10-on-windows-phone-8.html), this fix should work in Mobile IE10 on WP8s using GDR3. I have a preview of the update on my phone and it looks like phones are starting to get the full release now-ish as well.

I haven't had a chance to test this but figured this shouldn't hurt anything if it doesn't work and we can take a look at it some more later.