devel: update nettle submodule

This updates nettle submodule to the 3.9.1 release.

Signed-off-by: Daiki Ueno <ueno@gnu.org>

diff --git a/devel/nettle b/devel/nettle
index 7a5f86321f4c67d7219aa87ea4e2ddca677d7378..d2cc9b95b50440c331ee143312309951a7e8d7ca 160000 (submodule)
--- a/devel/nettle
+++ b/devel/nettle
@@ -1 +1 @@
-Subproject commit 7a5f86321f4c67d7219aa87ea4e2ddca677d7378
+Subproject commit d2cc9b95b50440c331ee143312309951a7e8d7ca