[BUGFIX] Fix disappearing table head in extension manager

This is a sass recompilation of the extension manager scss that fixes
the problem of disappearing table heads.

Change-Id: I7b708f34798714170bed1cbf1b76599990c9dd6f
Fixes: #50440
Releases: 6.2
Reviewed-on: https://review.typo3.org/22611
Reviewed-by: Markus Klein
Tested-by: Markus Klein