Add release checksums for opusfile 0.12.

We keep checksums of the release packages in the repo as a
redundant and trustworthy reference. I found these in an
old working repo; it looks like they weren't committed
after the 0.12 release last year.

Checked against the files on downloads.xiph.org.

releases.sha2

@@ -21,3 +21,5 @@ e9591da4d4c9e857436c2d46a28a9e470fa5355ea5a76d4d582f137d18755d36  opusfile-0.9.z
 9d9e95d01817ecf48bf6daaea8f071f9b45bd1751ca1fc8ce50e5075eb2bc3c8  opusfile-0.10.zip
 74ce9b6cf4da103133e7b5c95df810ceb7195471e1162ed57af415fabf5603bf  opusfile-0.11.tar.gz
 23c5168026c4f1fc34843650135b409d0fc8cf452508163b4ece8077256ac6ff  opusfile-0.11.zip
+118d8601c12dd6a44f52423e68ca9083cc9f2bfe72da7a8c1acb22a80ae3550b  opusfile-0.12.tar.gz
+7f44575596b78d7787c1865b9653e2a71546ff1ae77d87c53ab16dcc7af295ba  opusfile-0.12.zip