meson: use SPDX expression for license

This is the format recommended by Meson documentation.

build/meson/meson.build

@@ -10,7 +10,7 @@
 
 project('zstd',
   ['c', 'cpp'],
-  license: ['BSD', 'GPLv2'],
+  license: 'BSD-3-Clause OR GPL-2.0-only',
   default_options : [
     # There shouldn't be any need to force a C standard convention for zstd
     # but in case one would want that anyway, this can be done here.