build: update from gnulib, for fixed lib/signbitl.c

* gnulib: Update submodule to latest.

diff --git a/gnulib b/gnulib
index f56ff6b35bc3869a7d7c4191e06e79c1338caac3..278d609ead8d065b4c4a94705b8fd127c3df367a 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit f56ff6b35bc3869a7d7c4191e06e79c1338caac3
+Subproject commit 278d609ead8d065b4c4a94705b8fd127c3df367a