[PATCH] Fix swig-py bindings with SWIG 3.0.4

Hello,

> I saw build errors in the bindings with SWIG 3.0.4:
> ./subversion/bindings/swig/core.i:792: Error: Unknown SWIG preprocessor
> directive: The (if this is a block of target language code, delimit it
> with %{ and %})

And here is the missing part for proxy.swg for review...

[[[
Follow-up to 1655262,1658347: Fix for python bindings build with Swig 3.0.x

* subversion/bindings/swig/include/proxy.swg:
  Escape comments

Patch by: Dominique Leuenberger <dimstar_at_opensuse.org>
Found by: me
]]]

Andreas