system/Error: allow using errno functions on Windows

The Windows standard library emulates part of POSIX, which includes
errno.

src/system/Error.hxx

@@ -90,7 +90,7 @@ FormatLastError(const char *fmt, Args&&... args)
 			       std::forward<Args>(args)...);
 }
 
-#else
+#endif /* WIN32 */
 
 #include <errno.h>
 #include <string.h>
@@ -125,8 +125,6 @@ FormatErrno(const char *fmt, Args&&... args)
 	return FormatErrno(errno, fmt, std::forward<Args>(args)...);
 }
 
-#endif
-
 gcc_pure
 static inline bool
 IsFileNotFound(const std::system_error &e)