build: update gnulib submodule to latest

Mainly to avoid make dist-check failure with --disable-nls

diff --git a/gnulib b/gnulib
index caf768863e2a411ede373164e861b0bf6b707bcc..2481e7a50d6535582856626b53009f419e2e05e2 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit caf768863e2a411ede373164e861b0bf6b707bcc
+Subproject commit 2481e7a50d6535582856626b53009f419e2e05e2