graphics/py-pillow: Enable default support for TIFF images

Add the TIFF knob, which installs graphics/tiff and enables the bindings for
TIFF image support, to the default options.

The library libtiff.so from graphics/tiff is already installed implicitly
through graphics/openjpeg using the default option JPEG2000 but Pillow's own
main library, _imaging.so, is not linked against it.

Thus convert the TIFF option into a default one to gain real use of the
libtiff.so that has been already there.

Approved by: koobs (maintainer)