build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 4e6e16b3f43ce96302b1e52e48730c1f15e18c14..87c456bf6fb9453144890f0941c118a49b20fda5 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 4e6e16b3f43ce96302b1e52e48730c1f15e18c14
+Subproject commit 87c456bf6fb9453144890f0941c118a49b20fda5