liblzma: Add support for LZMA_SYNC_FLUSH in the Block encoder.

The documentation mentions that lzma_block_encoder() supports
LZMA_SYNC_FLUSH but it was never added to supported_actions[]
in the internal structure. Because of this, LZMA_SYNC_FLUSH could
not be used with the Block encoder unless it was the next coder
after something like stream_encoder() or stream_encoder_mt().

src/liblzma/common/block_encoder.c

@@ -217,6 +217,7 @@ lzma_block_encoder(lzma_stream *strm, lzma_block *block)
 	lzma_next_strm_init(lzma_block_encoder_init, strm, block);
 
 	strm->internal->supported_actions[LZMA_RUN] = true;
+	strm->internal->supported_actions[LZMA_SYNC_FLUSH] = true;
 	strm->internal->supported_actions[LZMA_FINISH] = true;
 
 	return LZMA_OK;