fix pdfium mutex init
fix pdfium mutex init

We need to make the mutex in _class_init, not _build, since we can lock
even if _build is not called.

lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips