Fix const struct to work with C++ compilation

This allows the libopus source files to be compiled with a C++ compiler,
by ensuring that the "extern const MLP net" declaration is visible at
the point of the "const MLP net" in src/mlp_data.c.  Otherwise it will
default to static linkage in C++, resulting in an undefined symbol.

Note that if compiled with a C++ compiler the symbols will be mangled,
so don't do that if you want to be able to call it from C code or run
the tests (which still require a C compiler).
diff --git a/src/mlp.h b/src/mlp.h
index 053b23b..618e246 100644
--- a/src/mlp.h
+++ b/src/mlp.h
@@ -36,6 +36,8 @@
     const float *weights;
 } MLP;
 
+extern const MLP net;
+
 void mlp_process(const MLP *m, const float *in, float *out);
 
 #endif /* _MLP_H_ */