[patch bugfix] Halt only on gb

[z9999]

When a water factory was built, it added a halt on every tile, regardless of whether on bg.
So, it can't be removed.
This patch will fix this problem.

Note:
This problem will happen only with fish_swarm on SF svn.
This problem doesn't happen on saved game, because it doesn't add halt on sea when loading.

[prissi]

Thank you, included.