Avoid unprefixed CamelCase headers generated directly in build dir
The removal of the PREFIX argument from ecm_generate_headers() call in 42a061b0 resulted in the headers being directly generated in the build dir, thus no longer only being visible via an explicit prefixed <KSyntaxHighlighting/*> include.
Alternative to !313 (merged) which just handles the bug and might need less discussion given 5.95 tagging upcoming WE