Remove warning when using KDEFrameworkCompilerSettings multiple times

(as can happen when having multiple frameworks as git submodules)

My first idea was to skip calling KDEClangFormat if not from the
toplevel of the project, but e.g. hotspot includes
KDEFrameworkCompilerSettings in the src subdir, not at the toplevel.
This fix is less intrusive, a single call will still work, even from
a subdirectory.