QmlFormat.cmake: avoid CMake policy warning CMP0175

CMake >= 3.31 complains about:

Exactly one of PRE_BUILD, PRE_LINK, or POST_BUILD must be given. Assuming POST_BUILD to preserve backward compatibility. [1]

PRE_BUILD and PRE_LINK are not recommended/available and we don't build anything here anyway, so just go with the past default POST_BUILD here.

[1] https://cmake.org/cmake/help/latest/policy/CMP0175.html