[BUGFIX] t3lib_div::validPathStr() fails on certain file names

Function t3lib_div::validPathStr() fails on file names with Unicode
characters. Adding 'u' modifier to the regular expression solves
the problem.

Change-Id: I9dfb31090d12133798f4e8d47e7394e6c7be51a6
Resolves: #32046
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/7032
Reviewed-by: Oliver Klee
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Stefan Neufeind
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch