graphics/dspdfviewer: partially unbreak with Clang 8

../dspdfviewer.cpp:311:48: error: empty expression statement has no effect; remove unnecessary ';' to silence this warning [-Werror,-Wextra-semi-stmt]

QSize newSize = window.getPreviewImageSize();;
                                             ^

PR: 236217
Approved by: portmgr blanket