allow dock water tile on aqueduct and buoy patch

[xarick]

Purpose of this patch is to ease the rules a bit when placing docks, in regards to the third tile check, the water tile.

v1:
- It allows construction if it is an aqueduct head.
- It allows construction if it is a buoy.

Note: This patch is similar to this one here viewtopic.php?f=33&t=75221 but I found out that some AIs are not prepared to deal with water tiles with one corner raised.