fix(segment): horizontal segments lost rounded corners

The horizontal segments should have the rounded corner, but the
rounded corners seem to be coverd by the background of the segment
inside of it.

The first and last segment inside the horizontal segments now adjust
the border-radius to appear the rounded corner from the horizontal
segments.

src/definitions/elements/segment.less

@@ -469,6 +469,12 @@
     .ui.horizontal.segments:not(.stackable) > .segment:first-child {
       border-left: none;
     }
+    .ui.horizontal.segments > .segment:first-child {
+      border-radius: @borderRadius 0 0 @borderRadius;
+    }
+    .ui.horizontal.segments > .segment:last-child {
+      border-radius: 0 @borderRadius @borderRadius 0;
+    }
   }
 }