extern(C++) compatibility with FILE*

There's probably other examples of this too in the druntime stdc bindings, but this is the current one that's hurting.

example:
D code: ``extern(C++) tree read_tree (FILE* finput);``
C++ code: ``tree func = read_tree (finput);``

Error: ``undefined reference to `read_tree(_IO_FILE*)'``
objdump D: ``_Z9read_treeP6_iobuf``
objdump C++: ``_Z9read_treeP8_IO_FILE``