Do not define stat if not necessary.

On recent mingw compiler, `stat` is already defined the same way we do.
So we don't need to do it.

src/server/kiwix-serve.cpp

@@ -138,7 +138,7 @@ void setup_sighandlers()
 
 uint64_t fileModificationTime(const std::string& path)
 {
-#if defined(_WIN32)
+#if defined(_WIN32) && !defined(stat)
 #define stat _stat
 #endif
   struct stat fileStatData;