urngd: Fix wrong type in format string

GCC 9.1 complains about this wrong type used in the format string, fix
this to make the compiler happy.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 9b53201d9c)

package/system/urngd/patches/001-fix-compile.patch

@@ -0,0 +1,11 @@
+--- a/urngd.c
++++ b/urngd.c
+@@ -114,7 +114,7 @@ static size_t gather_entropy(struct urng
+ 
+ 	ret = write_entropy(u, buf, sizeof(buf), ENTROPYBYTES);
+ 	if (sizeof(buf) != ret) {
+-		ERROR("injected %lub of entropy, less then %db expected\n",
++		ERROR("injected %zub of entropy, less then %db expected\n",
+ 		      ret, sizeof(buf));
+ 	} else {
+ 		ret = sizeof(buf);