[BUGFIX] Add .htaccess file to ext:extensionmanager/Resources/Private

Change-Id: I7ada1e5e40067cf1faed8b2b5d338a690e921cad
Fixes: #46264
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/18898
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn