[ksmserver] Use QUrl::fromUserInput to construct sound url

Since Phonon 4.9, this code fails to play a login sound if the sound
file is specified as absolute file path (without "file://"). The reason
is that QUrl() treats the parameter as Url (not as file path), and this
only accidentally worked with earlier Phonon versions but not any more.
This patch uses QUrl::fromUserInput() instead to create a QUrl from the
string in the settings file, which fixes the problem.

This is the same change as
https://phabricator.kde.org/R289:9db06adc8114163f401417064b07772139bc36bc
in knotifications.
A more detailed explanation of the problem can be found in
https://bugs.kde.org/show_bug.cgi?id=337276#c12 .

BUG: 392725
FIXED-IN: 5.12.5
Differential Revision: https://phabricator.kde.org/D12606