Add libsodium license, even though the source is not really included in this repo, update cinterop def to point to absolute path (still missingfor linker)

EXTERNAL_LICENSES

@@ -0,0 +1,23 @@
+Libraries used by multipatform-crypto-delegated follow:
+
+Libsodium, licensed under ISC License
+
+/*
+* ISC License
+*
+* Copyright (c) 2013-2020
+* Frank Denis <j at pureftpd dot org>
+*
+* Permission to use, copy, modify, and/or distribute this software for any
+* purpose with or without fee is hereby granted, provided that the above
+* copyright notice and this permission notice appear in all copies.
+*
+* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+