Refactoring in commit v3.7.0-rc1~291^2~1 (Refactor extra generator
registration to use factories, 2016-07-20) accidentally switched
the order of the "extra - base" generator names to "base - extra".
Switch it back.  While this could affect all callers of the
`GetRegisteredGenerators` method, only cmake-gui actually used this
particular field.

Closes: #16359