kdePackages.ksystemstats: cmakeFlags -> extraCmakeFlags

Just for consistency.

pkgs/kde/plasma/ksystemstats/default.nix

···
     libnl
   ];
 
-  cmakeFlags = [
+  extraCmakeFlags = [
     "-DSYSTEMSTATS_DBUS_INTERFACE=${libksysguard}/share/dbus-1/interfaces/org.kde.ksystemstats1.xml"
   ];
 }