Add missing includes to fix compile errors

fs.cpp:35:17: error: no member named 'strerror' in namespace 'std'
    return std::strerror(errno);
           ~~~~~^
fs.cpp:49:9: error: use of undeclared identifier 'close'
        close(fd);
        ^
2 errors generated.

./interfaces/chain.h:265:55: error: ‘std::function’ has not been declared
     virtual void rpcRunLater(const std::string& name, std::function<void()> fn, int64_t seconds) = 0;
                                                       ^~~

src/fs.cpp

@@ -5,10 +5,12 @@
 #include <fs.h>
 
 #ifndef WIN32
+#include <cstring>
 #include <fcntl.h>
 #include <string>
 #include <sys/file.h>
 #include <sys/utsname.h>
+#include <unistd.h>
 #else
 #ifndef NOMINMAX
 #define NOMINMAX
@@ -31,7 +33,8 @@ FILE *fopen(const fs::path& p, const char *mode)
 
 #ifndef WIN32
 
-static std::string GetErrorReason() {
+static std::string GetErrorReason()
+{
     return std::strerror(errno);
 }
 

src/interfaces/chain.h

@@ -8,6 +8,7 @@
 #include <optional.h>               // For Optional and nullopt
 #include <primitives/transaction.h> // For CTransactionRef
 
+#include <functional>
 #include <memory>
 #include <stddef.h>
 #include <stdint.h>