add omitted header

certain files needed to include either fstream and our shim for std::filesystem.
this includes fstream into our shim and includes this shim in places
that require fstream. this is done because some toolchains (cough
cough broke af arch linux amalgums) can have weird subsets of the
requirements of C++17 that overlap, except when they dont, denoted by
unknowable undisclosed circumstances.

this issue was reported by a user in the wild, and this fixes it.

llarp/bootstrap.cpp

@@ -2,6 +2,7 @@
 #include "util/bencode.hpp"
 #include "util/logging.hpp"
 #include "util/logging/buffer.hpp"
+#include "util/fs.hpp"
 
 namespace llarp
 {

llarp/util/fs.hpp

@@ -1,5 +1,5 @@
 #pragma once
-
+#include <fstream>
 #ifdef USE_GHC_FILESYSTEM
 #include <ghc/filesystem.hpp>
 namespace fs = ghc::filesystem;

llarp/vpn/linux.hpp

@@ -24,6 +24,8 @@
 #include <llarp/router/abstractrouter.hpp>
 #include <llarp.hpp>
 
+#include <llarp/util/fs.hpp>
+
 namespace llarp::vpn
 {
   struct in6_ifreq