RA Airstrip Limit Miscount

[ChthonVII]

Helipads are improperly counted towards the airstrip limit in ObjectTypeClass::Who_Can_Build_Me(). This allows you to create an extra plane if you have at least one helipad and at least one plane in the air at the moment construction of the extra plane completes. (And later, unless you're very good at juggling, one of your planes will crash due to not having somewhere to land.) Petroglyph made this worse by spamming retries every tick when production buildings are blocked. (See this commit.)