Fix copying of files with extended ACLs.

The switch to #cmakedefine01 in KDE Frameworks has detected a missing
 #include <config-acl.h> in file_unix.cpp ever since it was splitted
out. In case we still had a doubt about how much more superior the
cmakedefine01 (#if instead of #ifdef) solution is...

CCBUG: 191474
  (not tested, but might help...)