Feature: allow to do a hostile takeover of an AI company (in singleplayer) (#10914)

With the removal of the share-system, you could no longer make an
AI disappear in a single player game. At least, not without going
into the console.

src/company_base.h

@@ -165,6 +165,7 @@ struct Company : CompanyProperties, CompanyPool::PoolItem<&_company_pool> {
 };
 
 Money CalculateCompanyValue(const Company *c, bool including_loan = true);
+Money CalculateHostileTakeoverValue(const Company *c);
 
 extern uint _cur_company_tick_index;