cad/gmsh: Fix fetch


cad/gmsh: Fix fetch

Add the distfile to LOCAL/zeising as well, since I did the update.
For some reason the distfile is notoriously hard to fetch from upstream, so
add a local copy of it as well.

From the buildlog:

=> Attempting to fetch http://gmsh.info/src/gmsh-4.6.0-source.tgz
gmsh-4.6.0-source.tgz                                   11 MB 1542  Bps 02h10m
fetch: transfer timed out
fetch: gmsh-4.6.0-source.tgz appears to be truncated: 12047097/12048076 bytes
=> Attempting to fetch http://distcache.FreeBSD.org/ports-distfiles/gmsh/gmsh-4.6.0-source.tgz
fetch: http://distcache.FreeBSD.org/ports-distfiles/gmsh/gmsh-4.6.0-source.tgz: size mismatch: expected 12048076, actual 12047097
=> Couldn't fetch it - please try to retrieve this