tests: update gnulib to latest to avoid FP DST-related test failure

Otherwise, "make check" would fail after e.g., a CEST-to-CST
daylight savings transition.
See http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/28825

diff --git a/gnulib b/gnulib
index f1a5c91522554791317dc2ee763fe8c017c7b810..56ddf0fdeb52ce76718e0594db4f567401e90a2c 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit f1a5c91522554791317dc2ee763fe8c017c7b810
+Subproject commit 56ddf0fdeb52ce76718e0594db4f567401e90a2c