build: update gnulib submodule to latest

For the Gnulib commit 2c480fa522 (mbrtowc, mbrtoc32: Silence -Wshadow
warnings (regr. 2026-04-02)., 2026-04-06).

diff --git a/gnulib b/gnulib
index 305e58f2f803035bccfe051c29d8ffd3d13dfdbc..1039a5f2cee3cda1c11f64a5eb3a15b2e87cd2f0 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 305e58f2f803035bccfe051c29d8ffd3d13dfdbc
+Subproject commit 1039a5f2cee3cda1c11f64a5eb3a15b2e87cd2f0