qtimageformats: cmake fails if libwebp is built with cmake

CMake Error at /usr/lib/cmake/Qt6/QtPublicTargetHelpers.cmake:260 (set_property):
  Attempt to promote imported target "Threads::Threads" to global scope (by setting IMPORTED_GLOBAL) which is not built in this directory.
Call Stack (most recent call first):
  /usr/lib/cmake/Qt6/QtPublicWalkLibsHelpers.cmake:255 (__qt_internal_promote_target_to_global)
  /usr/lib/cmake/Qt6/QtPublicWalkLibsHelpers.cmake:228 (__qt_internal_walk_libs)
  /usr/lib/cmake/Qt6/QtFindPackageHelpers.cmake:15 (__qt_internal_walk_libs)
  /usr/lib/cmake/Qt6/QtFindPackageHelpers.cmake:200 (qt_find_package_promote_targets_to_global_scope)
  src/imageformats/configure.cmake:22 (qt_find_package)
  src/plugins/imageformats/CMakeLists.txt:12 (include)