[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/cmake/lib/CMakeLists.txt

@@ -162,8 +162,8 @@ endif ()
 # install target
 install(FILES
     "${LIBRARY_DIR}/zstd.h"
-    "${LIBRARY_DIR}/dictBuilder/zdict.h"
-    "${LIBRARY_DIR}/common/zstd_errors.h"
+    "${LIBRARY_DIR}/zdict.h"
+    "${LIBRARY_DIR}/zstd_errors.h"
     DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}")
 
 install(TARGETS ${library_targets}