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/wallet/db.cpp

@@ -4,8 +4,8 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <chainparams.h>
-#include <fs.h>
 #include <logging.h>
+#include <util/fs.h>
 #include <util/system.h>
 #include <wallet/db.h>