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