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).