KSycoca: avoid database rebuild if XDG_CONFIG_DIRS contains duplicates
Similar to ffbfbba0.
We compare extraFiles.keys() to KBuildSycoca::factoryExtraFiles() to see if there are any files added.
As extraFiles.keys is from a map, duplicates are implicitly removed. This is not true for the stringlist.
Duplicates can arise if a user has the same directory twice in XDG_CONFIG_DIRS or in XDG_CURRENT_DESKTOP.