fix: include <fcntl.h> for O_RDONLY on Windows/MSVC

[ZeZezhi]

Problem

Building libhv with MSVC on Windows fails in http/server/FileCache.cpp:

error C2065: "O_RDONLY": undeclared identifier

O_RDONLY is declared in <fcntl.h>, which was not included. It compiled on Linux only because another header happened to pull it in transitively.

Fix

Add #include <fcntl.h>.

Verification

• OS: Windows 11, MSVC (VS 2022), Ninja + CMake
• Full build cmake --build . now succeeds (134/134 targets)
• Discovered while building the project for the first time