[DynamicLibrary] Fix build on musl libc

Summary:
On musl libc, stdin/out/err are defined as `FILE* const` globals,
and their address is not implicitly convertible to void *,
or at least gcc 6 doesn't allow it, giving errors like:

```
error: cannot initialize return object of type 'void *' with an rvalue of type 'FILE *const *' (aka '_IO_FILE *const *')
    EXPLICIT_SYMBOL(stderr);
    ^~~~~~~~~~~~~~~~~~~~~~~
```

Add an explicit cast to fix that problem.

Reviewers: marsupial, krytarowski, dim
Reviewed By: dim
Differential Revision: https://reviews.llvm.org/D39297

llvm-svn: 316672