[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.

contrib/VS2005/zstdlib/zstdlib.vcproj

@@ -446,7 +446,7 @@
 				>
 			</File>
 			<File
-				RelativePath="..\..\..\lib\common\zstd_errors.h"
+				RelativePath="..\..\..\lib\zstd_errors.h"
 				>
 			</File>
 			<File
@@ -478,7 +478,7 @@
 				>
 			</File>
 			<File
-				RelativePath="..\..\..\lib\dictBuilder\zdict.h"
+				RelativePath="..\..\..\lib\zdict.h"
 				>
 			</File>
 			<File