Fix KDGantt header being recreated on every cmake run

[dfaure-kdab]

COMMON_HEADER already takes care of creating it, no need to do it again with configure_file

[dantti]

LGTM but I wonder what COMMON_HEADER generates, as maybe the list(APPEND) could also be removed?

[dfaure-kdab]

I added a message() to debug it, and ecm_generate_headers puts all the forwarding headers into kdchart_HEADERS, but nothing else. This is why the install(FILES) below has a manual list of all the real *.h headers to install, in addition to the forwarding headers.

This means we could also just add the (generated) kdgantt.h in the install(FILES command, instead of doing list(APPEND). It's basically the same though.