Migrate to SPDX license

This is a mess. There is no sign of BSD - as far as source git allowed me to go. The frontpage at Savanah claims that the license is X11. But the license file and file headers refer to plain MIT.
This is part of https://fedoraproject.org/wiki/Changes/SPDX_Licenses_Phase_2

libunwind.spec

@@ -47,7 +47,7 @@ Summary: An unwinding library
 Name: libunwind
 Version: 1.7.2
 Release: 1%{?dist}
-License: BSD
+License: MIT
 URL: http://savannah.nongnu.org/projects/libunwind
 Source: https://github.com/libunwind/libunwind/releases/download/v%{version}/%{name}-%{version}.tar.gz