Replace includes of boost/filesystem.h with fs.h

This is step one in abstracting the use of boost::filesystem.

src/validation.h

@@ -13,6 +13,7 @@
 #include "amount.h"
 #include "chain.h"
 #include "coins.h"
+#include "fs.h"
 #include "protocol.h" // For CMessageHeader::MessageStartChars
 #include "script/script_error.h"
 #include "sync.h"
@@ -30,7 +31,6 @@
 #include <atomic>
 
 #include <boost/unordered_map.hpp>
-#include <boost/filesystem/path.hpp>
 
 class CBlockIndex;
 class CBlockTreeDB;