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/dbwrapper.cpp

@@ -4,10 +4,10 @@
 
 #include <dbwrapper.h>
 
-#include <fs.h>
 #include <logging.h>
 #include <random.h>
 #include <tinyformat.h>
+#include <util/fs.h>
 #include <util/fs_helpers.h>
 #include <util/strencodings.h>