refactor: Move fs.* to util/fs.*

The fs.* files are already part of the libbitcoin_util library. With the
introduction of the fs_helpers.* it makes sense to move fs.* into the
util/ directory as well.

src/flatfile.h

@@ -8,8 +8,8 @@
 
 #include <string>
 
-#include <fs.h>
 #include <serialize.h>
+#include <util/fs.h>
 
 struct FlatFilePos
 {