Fix building synctex 1.19 on Windows

Summary:
The massive changes to the synctex code in D7594 pulled in a new library
(on windows only), reflect that in the linker flags.

Test Plan:
craft okular on Okular's master branch builds successfully with MSVC
2017 and Qt 5.9, build on Linux still fine.

Reviewers: Okular, aacid

Reviewed By: Okular, aacid

Tags: Okular

Differential Revision: https://phabricator.kde.org/D7809