renamed zstd_compress.h into zstd_compress_internal.h

to emphasize the fact that all definitions it contains
must remain private, accross lib/compress modules.

lib/compress/zstd_fast.c

@@ -8,6 +8,7 @@
  * You may select, at your option, one of the above-listed licenses.
  */
 
+#include "zstd_compress_internal.h"
 #include "zstd_fast.h"