build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 875461ffdf58ac04677957b4ae4160465b83b940..6f6ca19e6cec9fec055258b1459108ef8822fe75 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 875461ffdf58ac04677957b4ae4160465b83b940
+Subproject commit 6f6ca19e6cec9fec055258b1459108ef8822fe75