graphics/darktable: Fix bug with preferences when built with Clang 3.6

The patch was committed upstream. The commit message has a detailed
explanation:
https://github.com/darktable-org/darktable/commit/9d77a28e54f8bc19592a170b1e4b9cf083f7b5a3

Differential Revision: https://reviews.freebsd.org/D2308
Reviewed by: kwm
Approved by: kwm