build: update gnulib to latest, for memmem/strstr fix

diff --git a/gnulib b/gnulib
index 2bb63bfb25474ea147ee9f1523c0337997359a4c..c823199df2cc03b6bd70d0a2fef5999af82792fe 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 2bb63bfb25474ea147ee9f1523c0337997359a4c
+Subproject commit c823199df2cc03b6bd70d0a2fef5999af82792fe