graphics/darktable: bump PORTREVISION after fixes

Especially d8e040b64325 contains pkg-plist fixes, so
we need to make sure to flush out the original 3.6.0 packages.

Note for those triggering builds manually, there could be
another PORTREVISION bump due to (im)pending OpenEXR updates
within the next few hours.

Aligned with: Jean-Sébastien Pédron (dumbbell@, maintainer)