[1.5.0] Move zstd_errors.h and zdict.h to lib/ root

`zstd_errors.h` and `zdict.h` are public headers, so they deserve to be
in the root `lib/` directory with `zstd.h`, not mixed in with our private
headers.

build/meson/lib/meson.build

@@ -124,5 +124,5 @@ pkgconfig.generate(libzstd,
   url: 'http://www.zstd.net/')
 
 install_headers(join_paths(zstd_rootdir, 'lib/zstd.h'),
-  join_paths(zstd_rootdir, 'lib/dictBuilder/zdict.h'),
-  join_paths(zstd_rootdir, 'lib/common/zstd_errors.h'))
+  join_paths(zstd_rootdir, 'lib/zdict.h'),
+  join_paths(zstd_rootdir, 'lib/zstd_errors.h'))