convert license to SPDX

This is part of https://fedoraproject.org/wiki/Changes/SPDX_Licenses_Phase_4

vamp-plugin-sdk.spec

@@ -9,6 +9,7 @@ Summary:        An API for audio analysis and feature extraction plugins
 %global gittag %{name}-v%{version}
 
 # KissFFT code in src/vamp-sdk/ext is BSD-3-Clause
+# Automatically converted from old format: MIT AND BSD-3-Clause - review is highly recommended.
 License:        MIT AND BSD-3-Clause
 URL:            https://vamp-plugins.org/
 Source:         https://github.com/vamp-plugins/vamp-plugin-sdk/archive/%{gittag}/%{name}-%{version}.tar.gz