liblzma: Fix compilation of fastpos_tablegen.c.

The macro lzma_attr_visibility_hidden has to be defined to make
fastpos.h usable. The visibility attribute is irrelevant to
fastpos_tablegen.c so simply #define the macro to an empty value.

fastpos_tablegen.c is never built by the included build systems
and so the problem wasn't noticed earlier. It's just a standalone
program for generating fastpos_table.c.

Fixes: https://github.com/tukaani-project/xz/pull/69
Thanks to GitHub user Jamaika1.

src/liblzma/lzma/fastpos_tablegen.c

@@ -13,6 +13,8 @@
 
 #include <inttypes.h>
 #include <stdio.h>
+
+#define lzma_attr_visibility_hidden
 #include "fastpos.h"