build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 959d9cb463d992f9c5cc2fbeb54bf12eee2346f7..791cc509ac459a2555f8d633ad67455cf8d3fe4d 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 959d9cb463d992f9c5cc2fbeb54bf12eee2346f7
+Subproject commit 791cc509ac459a2555f8d633ad67455cf8d3fe4d