Add box-sizing attribute to segmented control

On the monolith we have a base css file somewhere that adds this
property by default. In this package we don't (and even if we did, we
couldn't bundle it with the elm code), so this commit adds the property
manually on a div that needs it.

src/Nri/Ui/SegmentedControl/V1.elm

@@ -91,6 +91,7 @@ styles =
             , border3 (px 1) solid Colors.azure
             , borderLeft (px 0)
             , borderBottom3 (px 3) solid Colors.azure
+            , boxSizing borderBox
             ]
         , Css.Foreign.class Focused
             [ color Colors.gray20