-base svn compilaton fails in NSJSONSerialization.m

[Sebastian Reitenbach]

I guess the latest changes to the NSXML stuff in -base did not make it compile 
anymore:

 Compiling file NSJSONSerialization.m ...
In file included from ../Headers/Foundation/Foundation.h:133:0,
                 from NSJSONSerialization.m:10:
../Headers/Foundation/NSXMLDocument.h:84:20: error: duplicate member ‘_URI’
In file included from ../Headers/Foundation/Foundation.h:134:0,
                 from NSJSONSerialization.m:10:
../Headers/Foundation/NSXMLDTD.h:49:18: error: duplicate member ‘_children’
make[4]: *** [obj/libgnustep-base.obj/NSJSONSerialization.m.o] Error 1

make[3]: *** [internal-library-all_] Error 2
make[2]: *** [libgnustep-base.all.library.variables] Error 2
make[1]: *** [internal-all] Error 2
make: *** [internal-all] Error 2