build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 46f9c21a08245fe224fd975de8632b04a0256387..78b62f8320f8dffb813222c1480563ed14251ca3 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 46f9c21a08245fe224fd975de8632b04a0256387
+Subproject commit 78b62f8320f8dffb813222c1480563ed14251ca3