Fix build with Qt 5.15 by adding an include

Summary:
QPainterPath is no longer included via qtransform.h (since
5.15.0-beta2, 50d2acdc93b4de2ba56eb67787e2bdcb21dd4bea in qtbase.git).

Depends on D28255.

Test Plan: Builds fine against 5.15.0-beta2 where it previously didn't

Reviewers: aacid

Differential Revision: https://phabricator.kde.org/D28256