added ZSTD_sizeofDCtx()

lib/decompress/zstd_decompress.c

@@ -135,7 +135,7 @@ struct ZSTD_DCtx_s
     BYTE headerBuffer[ZSTD_FRAMEHEADERSIZE_MAX];
 };  /* typedef'd to ZSTD_DCtx within "zstd_static.h" */
 
-size_t ZSTD_sizeofDCtx (void) { return sizeof(ZSTD_DCtx); }   /* non published interface */
+size_t ZSTD_sizeofDCtx (const ZSTD_DCtx* dctx) { return sizeof(*dctx); }
 
 size_t ZSTD_decompressBegin(ZSTD_DCtx* dctx)
 {