[BUGFIX] Fixes wrong SQL in ext_tables.sql

Resolves: #54794
Releases: 3.7, 3.8

Change-Id: I18c9eb76d689c0c131f276a2616ad1f5e06a5e4b
Reviewed-on: https://review.typo3.org/26666
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee