Fix build failure due to undeclared xrdShell
Fix a build failure (issue #9 (closed)) due to xrdShell being undeclared in VRMirror.cpp since commit c79a508a ("update to kwin 5.25 and clean up GLTexture fork").
It now builds for me, however I haven't yet tested successfully due to invalid view configuration type error (monado). Hopefully thats unrelated... Sanity checking requested.
Link: #9 (closed) Fixes: c79a508a ("update to kwin 5.25 and clean up GLTexture fork")