multimedia/dvdauthor: Fails to build if GraphicsMagick is installed

Switch to using latest commit from github. The handling of the image magic dependency has been
improved in [1] in 2017, but the software has not seen a release since.

[1] https://github.com/ldo/dvdauthor/commit/a0d57bebb3a5d7259a0508a07a898cfe044155a1

PR: 240270
Submitted by: VVD <vvd@unislabs.com>
Reported by: Ting-Wei Lan <lantw44@gmail.com>