fix eductional decoder

fix #1774
also :
- fix minor compilation warnings
- make sure the `test` is run during CI tests

doc/educational_decoder/zstd_decompress.h

@@ -7,6 +7,8 @@
  * in the COPYING file in the root directory of this source tree).
  */
 
+#include <stddef.h>   /* size_t */
+
 /******* EXPOSED TYPES ********************************************************/
 /*
 * Contains the parsed contents of a dictionary
@@ -39,7 +41,7 @@ size_t ZSTD_get_decompressed_size(const void *const src, const size_t src_len);
  * Return a valid dictionary_t pointer for use with dictionary initialization
  * or decompression
  */
-dictionary_t* create_dictionary();
+dictionary_t* create_dictionary(void);
 
 /*
  * Parse a provided dictionary blob for use in decompression