synapticstouchpad.cpp: Drop synaptics-properties.h parent dir from include

Summary: Otherwise, this does not get the header from Synaptics_INCLUDE_DIRS.

Test Plan: Still builds.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29215