Add missing license field required by cargo deb.

successor/Cargo.toml

@@ -4,6 +4,9 @@ name = "successor"
 version = "0.1.0"
 edition = "2021"
 description = "Long enough for Lintian, short enough and without period for rpmlint"
+# Per https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields
+# this must be one of the short IDs at https://github.com/spdx/license-list-data/blob/v3.11/licenses.md
+license = "BSD-3-Clause"
 
 [dependencies]