Make KFileMetaData optional again

In 15b20cd0, I added KFileMetaData to
the required build dependencies in order to make the version increase
in 6c5930c2 effective, and ensure that
the build fails at the CMake stage if the KFileMetaData version is too
old.

However, I had missed that KFileMetaData was already an optional
dependency. This commit makes it optional again and adjusts the
required version.

Thanks to Christoph Feck for notifying me about this issue and sorry
for the stupid mistake.

CCMAIL: cfeck@kde.org