[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/VS2010/libzstd/libzstd.vcxproj

@@ -61,7 +61,7 @@
     <ClInclude Include="..\..\..\lib\common\threading.h" />
     <ClInclude Include="..\..\..\lib\common\bitstream.h" />
     <ClInclude Include="..\..\..\lib\common\error_private.h" />
-    <ClInclude Include="..\..\..\lib\common\zstd_errors.h" />
+    <ClInclude Include="..\..\..\lib\zstd_errors.h" />
     <ClInclude Include="..\..\..\lib\common\mem.h" />
     <ClInclude Include="..\..\..\lib\common\fse.h" />
     <ClInclude Include="..\..\..\lib\common\huf.h" />