Add license notice to API headers

[theStack]

This PR attempts to resolve #412 in a pragmatic way, adding the same MIT license block we use in internal files (see e.g. in the most recent modules ellswift, musig and silentpayments) for the public headers. Depending on how nit-picky we are one could further refine this, e.g.:

• add a copyright line (maybe in Bitcoin Core's style, e.g. Copyright (c) 2013-present The libsecp256k1 developers, with different starting years depending on when a module was developed/merged, or even without any year)?
• use SPDX license identifiers (e.g. /* SPDX-License-Identifier: MIT */) for machine readability?
• avoid mentioning the COPYING file since it's not installed? (there is still the URL though)

Probably it's not worth it to spend a lot of time on this; I guess one way to "solve" this is also to simplify leave as-is (and then still close #412 referring to that decision), but it seems at least quite common that public headers in C libraries contain some minimum copyright/licensing information.