liblzma: Add the forgotten lzma_lzma2_block_size().

This should have been in 5eefc0086d.

src/liblzma/lzma/lzma2_encoder.c

@@ -391,3 +391,13 @@ lzma_lzma2_props_encode(const void *options, uint8_t *out)
 
 	return LZMA_OK;
 }
+
+
+extern uint64_t
+lzma_lzma2_block_size(const void *options)
+{
+	const lzma_options_lzma *const opt = options;
+
+	// Use at least 1 MiB to keep compression ratio better.
+	return my_max((uint64_t)(opt->dict_size) * 3, UINT64_C(1) << 20);
+}

src/liblzma/lzma/lzma2_encoder.h

@@ -38,4 +38,6 @@ extern uint64_t lzma_lzma2_encoder_memusage(const void *options);
 
 extern lzma_ret lzma_lzma2_props_encode(const void *options, uint8_t *out);
 
+extern uint64_t lzma_lzma2_block_size(const void *options);
+
 #endif