[libzstd] Add ZSTD_c_literalCompressionMode flag

It controls the literals compression. It is either
`auto`, `huffman`, or `uncompressed`. It defaults to
`auto`, which is the current behavior.

lib/compress/zstd_compress_internal.h

@@ -188,6 +188,7 @@ struct ZSTD_CCtx_params_s {
                                 * 1<<wLog, even for dictionary */
 
     ZSTD_dictAttachPref_e attachDictPref;
+    ZSTD_literalCompressionMode_e literalCompressionMode;
 
     /* Multithreading: used to pass parameters to mtctx */
     int nbWorkers;