Windows is much pickier about dllimport/dllexport. Declare it on
the declaration, not the definition. Also ensure that the declaration
precedes the definition. Finally, remove a stray OPENSSL_EXPORT.

Change-Id: Id50b9de5acbe5adf1b15b22dd60b7a5c13a80cce
Reviewed-on: https://boringssl-review.googlesource.com/1862
Reviewed-by: Adam Langley <agl@google.com>