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/fs.h

@@ -9,6 +9,7 @@
 
 #include <cstdio>
 #include <filesystem>
+#include <functional>
 #include <iomanip>
 #include <ios>
 #include <ostream>
@@ -199,6 +200,7 @@ bool create_directories(const std::filesystem::path& p, std::error_code& ec) = d
 
 /** Bridge operations to C stdio */
 namespace fsbridge {
+    using FopenFn = std::function<FILE*(const fs::path&, const char*)>;
     FILE *fopen(const fs::path& p, const char *mode);
 
     /**

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;

src/validation.h

@@ -1014,13 +1014,11 @@ bool DeploymentEnabled(const ChainstateManager& chainman, DEP dep)
     return DeploymentEnabled(chainman.GetConsensus(), dep);
 }
 
-using FopenFn = std::function<FILE*(const fs::path&, const char*)>;
-
 /** Dump the mempool to disk. */
-bool DumpMempool(const CTxMemPool& pool, const fs::path& dump_path, FopenFn mockable_fopen_function = fsbridge::fopen, bool skip_file_commit = false);
+bool DumpMempool(const CTxMemPool& pool, const fs::path& dump_path, fsbridge::FopenFn mockable_fopen_function = fsbridge::fopen, bool skip_file_commit = false);
 
 /** Load the mempool from disk. */
-bool LoadMempool(CTxMemPool& pool, CChainState& active_chainstate, FopenFn mockable_fopen_function = fsbridge::fopen);
+bool LoadMempool(CTxMemPool& pool, CChainState& active_chainstate, fsbridge::FopenFn mockable_fopen_function = fsbridge::fopen);
 
 /**
  * Return the expected assumeutxo value for a given height, if one exists.