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_ldm.h

@@ -10,12 +10,13 @@
 #ifndef ZSTD_LDM_H
 #define ZSTD_LDM_H
 
-#include "zstd_compress.h"
-
 #if defined (__cplusplus)
 extern "C" {
 #endif
 
+#include "zstd_compress_internal.h"   /* ldmParams_t, U32 */
+#include "zstd.h"   /* ZSTD_CCtx, size_t */
+
 /*-*************************************
 *  Long distance matching
 ***************************************/