Lower Qt requirement back to Qt 5.2
Lower Qt requirement back to Qt 5.2

This got changed in commit d5d89fd2, but the commit message doesn't
explain any reason for this change. In addition, there has been no real
changes in this module since that commit, so it is very unlikely that I
missed something which started depending on Qt 5.4+.

KDE/gpgmepp,KDE/gpgmepp,KDE/gpgmepp