games/flightgear: disable the ninja generator

• with ninja 1.9 duplicate targets are errors [1]
• disable ninja until the issue [2] is fixed upstream.

[1] https://github.com/ninja-build/ninja/commit/42eefc854b32b3c1efc27548baaacdebad199853
[2] https://sourceforge.net/p/flightgear/codetickets/2118/

PR: 235507