[BUGFIX] Search word markup for UTF8 characters in summary

Fix usage of preg_split to use the modifier /u so that search words
are correctly split in a summary.

Resolves: #26172
Releases: master, 6.2, 6.1
Change-Id: I90d8676f5724f194434811378eb8598eaf04cb9e
Reviewed-on: http://review.typo3.org/20269
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>