There's a few things that will be kind of a nuisance and possibly not worth it
(crypto/asn1 dumps a lot of undeclared things, etc.). But it caught some
mistakes. Even without the warning, making sure to include the externs before
defining a function helps catch type mismatches.

Change-Id: I3dab282aaba6023e7cebc94ed7a767a5d7446b08
Reviewed-on: https://boringssl-review.googlesource.com/6484
Reviewed-by: Adam Langley <agl@google.com>