fix(broadcast message): adjust vertical spacing

Fixes a spacing issue mentioned in gitlab-org/gitlab-services/design.gitlab.com!1676 (comment 275501663):

even though the icon is 16x16 and the text is using 16px line height, the actual heights of those divs are 19px which is causing the spacing to look off

This MR changes the icon's vertical alignment to ensure that the 16px line-height is preserved.

Before	After