examples: hello-objc-gnustep: Fix 'make distclean'.
* gettext-tools/examples/hello-objc-gnustep/GNUmakefile (internal-distclean):
Remove the *.lproj directories.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile (internal-clean):
Remove remove-potcdate.sed.
