fix(GlIcon): Give icons img role

mentioned in commit 91f6cb7d

mentioned in merge request gitlab!62290 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2383 (merged)

mentioned in merge request status-page!291 (merged)

mentioned in merge request status-page!292 (merged)

mentioned in merge request gitlab-docs!1826 (merged)

mentioned in merge request gitlab-docs!1828 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2391 (merged)

mentioned in merge request status-page!296 (merged)

mentioned in merge request gitlab!62585 (merged)

mentioned in merge request gitlab-docs!1841 (merged)