[BUGFIX] Use correct IN statement for webmounts

This patch fixes the IN statement used for fetching the webmounts.
Previously, the statement looked like `IN("1,2,3")`, resulting in only
returning the last one.

Resolves: #76861
Related: #75546
Releases: master
Change-Id: I02204f5469d996e8ea967485737496b32a343231
Reviewed-on: https://review.typo3.org/48774


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>