Fix "type qualifiers ignored on function return type" errors.

Change-Id: If0dbbadb33a073b4faee500fdff900a5094ec889
Signed-off-by: Piotr Sikora <piotr@cloudflare.com>
Reviewed-on: https://boringssl-review.googlesource.com/1362
Reviewed-by: Adam Langley <agl@google.com>