os/FileSystem: disable mkfifo() wrapper on WIN32

src/fs/FileSystem.hxx

@@ -116,12 +116,16 @@ static inline bool RemoveFile(const Path &file)
  */
 Path ReadLink(const Path &path);
 
+#ifndef WIN32
+
 static inline bool
 MakeFifo(const Path &path, mode_t mode)
 {
 	return mkfifo(path.c_str(), mode) == 0;
 }
 
+#endif
+
 /**
  * Wrapper for access() that uses #Path names.
  */