config.mak.uname: Define FREAD_READS_DIRECTORIES for GNU/Hurd

GNU/Hurd is another platform that behaves like this. Set it to
UnfortunatelyYes so that config directory files are correctly processed.
This fixes the corresponding 'proper error on directory "files"' test in
t1308-config-set.sh.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -308,6 +308,7 @@ ifeq ($(uname_S),GNU)
 	NO_STRLCPY = YesPlease
 	HAVE_PATHS_H = YesPlease
 	LIBC_CONTAINS_LIBINTL = YesPlease
+	FREAD_READS_DIRECTORIES = UnfortunatelyYes
 endif
 ifeq ($(uname_S),IRIX)
 	NO_SETENV = YesPlease