fix gcc-10 strict aliasing warnings

by exposing HUF_CElt declaration.

lib/compress/huf_compress.c

@@ -102,11 +102,6 @@ static size_t HUF_compressWeights (void* dst, size_t dstSize, const void* weight
 }
 
 
-struct HUF_CElt_s {
-  U16  val;
-  BYTE nbBits;
-};   /* typedef'd to HUF_CElt within "huf.h" */
-
 /*! HUF_writeCTable() :
     `CTable` : Huffman tree to save, using huf representation.
     @return : size of saved CTable */