Add missing include in fs.cpp

Summary: This causes the build to fail with recent MinGw compilers.

Test Plan: Build with x86_64-w64-mingw32-gcc (GCC) 12.2.0

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13724