Fix build with Qt 5.14
Summary:
std::hash<QString> is already specialized in Qt 5.14 [1], so we need
to avoid redifining it with this version.
[1] 4469e36d7203a55a4e158a50f0e9effc3f2fa3c2 in qtbase.git
Test Plan: Builds fine with Qt 5.14.0-beta3
Reviewers: KDE PIM
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D25356