Add missing fs.h includes

The inclusion of this header should not depend on the inclusion of other
headers that include fs.h themselves.

src/node/chainstate.cpp

@@ -16,6 +16,7 @@
 #include <tinyformat.h>
 #include <txdb.h>
 #include <uint256.h>
+#include <fs.h>
 #include <util/time.h>
 #include <util/translation.h>
 #include <validation.h>