Fix #10048: Don't relocate company HQ on the same exact location

[glx22]

Motivation / Problem

A company's HQ can be relocated to the same exact location it already is in, costing the same amount (1% of value) as relocating to another location.

Description

Return "already built" error when trying to relocate company's HQ on the same exact location it already is in.
This doesn't affect the possibility to slightly move it over the current location.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[nielsmh]

LGTM