changed initStatic?Dict() return type to const ZSTD_?Dict*

ZSTD_create?Dict() is required to produce a ?Dict* return type
because `free()` does not accept a `const type*` argument.
If it wasn't for this restriction, I would have preferred to create a `const ?Dict*` object
to emphasize the fact that, once created, a dictionary never changes
(hence can be shared concurrently until the end of its lifetime).

There is no such limitation with initStatic?Dict() :
as stated in the doc, there is no corresponding free() function,
since `workspace` is provided, hence allocated, externally,
it can only be free() externally.

Which means, ZSTD_initStatic?Dict() can return a `const ZSTD_?Dict*` pointer.

Tested with `make all`, to catch initStatic's users,
which, incidentally, also updated zstd.h documentation.

lib/compress/zstd_compress.c

@@ -2634,7 +2634,8 @@ size_t ZSTD_freeCDict(ZSTD_CDict* cdict)
  *  Note : there is no corresponding "free" function.
  *         Since workspace was allocated externally, it must be freed externally.
  */
-ZSTD_CDict* ZSTD_initStaticCDict(void* workspace, size_t workspaceSize,
+const ZSTD_CDict* ZSTD_initStaticCDict(
+                                 void* workspace, size_t workspaceSize,
                            const void* dict, size_t dictSize,
                                  ZSTD_dictLoadMethod_e dictLoadMethod,
                                  ZSTD_dictMode_e dictMode,