closures: Add a missing include

Fixes building in userspace.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

lib/closure.c

@@ -9,6 +9,7 @@
 #include <linux/closure.h>
 #include <linux/debugfs.h>
 #include <linux/export.h>
+#include <linux/rcupdate.h>
 #include <linux/seq_file.h>
 #include <linux/sched/debug.h>