[BUGFIX] Show RTE tool tips in Internet Explorer

This patch adds a font size to the tool tip header class to show tool
tips in Internet Explorer 8 and 9 again.

Resolves: #63739
Releases: master, 6.2
Change-Id: Ib82190cbfc11dc9b51f7c0bb447b3ae9ae45a8c9
Reviewed-on: http://review.typo3.org/35239
Reviewed-by: Jo Hasenau <info@cybercraft.de>
Tested-by: Jo Hasenau <info@cybercraft.de>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>