[BUGFIX] Set dimensions for custom svg TYPO3 backend logo

Width and height is automatically set for pixel based backend
logos. This patch makes sure this also is done for svg file format.

Releases: master, 7.6
Resolves: #78590
Change-Id: Iff1dddc827d002e10fd4ec6f3b57bc16920b1c86
Reviewed-on: https://review.typo3.org/50522


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>