build: update gnulib submodule to latest

This includes a tweak to support building
the gnulib base32 module with -Wsuggest-attribute=const

diff --git a/gnulib b/gnulib
index a89e3449698f97ea0f9c5fbd73aa054009858471..7ab64d2610ca5aa78006bef91091dfa0268adf64 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit a89e3449698f97ea0f9c5fbd73aa054009858471
+Subproject commit 7ab64d2610ca5aa78006bef91091dfa0268adf64