Move FopenFn to fsbridge namespace

[META] In a future commit in this patchset, it will be used by more than
       just validation, and it needs to align with fopen anyway.

src/validation.cpp

@@ -17,6 +17,7 @@
 #include <consensus/validation.h>
 #include <cuckoocache.h>
 #include <flatfile.h>
+#include <fs.h>
 #include <hash.h>
 #include <kernel/coinstats.h>
 #include <logging.h>
@@ -64,6 +65,7 @@ using kernel::CCoinsStats;
 using kernel::CoinStatsHashType;
 using kernel::ComputeUTXOStats;
 
+using fsbridge::FopenFn;
 using node::BLOCKFILE_CHUNK_SIZE;
 using node::BlockManager;
 using node::BlockMap;