build: update gnulib submodule to latest

Mainly for
* 77d36b5 obstack: pacify GCC 6 with -Wnull-dereference

diff --git a/gnulib b/gnulib
index 6835fc458f30b94f15d69c35a79cbc2dfabe2d06..348402f2aac342bc925b7aaea9ee3cc353f427a9 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 6835fc458f30b94f15d69c35a79cbc2dfabe2d06
+Subproject commit 348402f2aac342bc925b7aaea9ee3cc353f427a9