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/walletdb.cpp

@@ -5,12 +5,12 @@
 
 #include <wallet/walletdb.h>
 
-#include <fs.h>
 #include <key_io.h>
 #include <protocol.h>
 #include <serialize.h>
 #include <sync.h>
 #include <util/bip32.h>
+#include <util/fs.h>
 #include <util/system.h>
 #include <util/time.h>
 #include <util/translation.h>