cad/gmsh: Update distinfo after upstream rolled a new tarball without a new
release

As far as I can tell, the only change is the name of the top-level
directory (WRKSRC). I do not have a copy of the old tarball to be certain,
and my email to the Gmsh list requesting information about the change has
not yet received a response.

Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D8432