lib: fix signed overflow warning in nl_object_diff()

Coverity warns "Signed integer overflow for expression '1<<31'".
diff --git a/lib/object.c b/lib/object.c
index 234628a..9b147d6 100644
--- a/lib/object.c
+++ b/lib/object.c
@@ -399,7 +399,7 @@
 	diff = nl_object_diff64(a, b);
 
 	return (diff & ~((uint64_t) 0xFFFFFFFF))
-		? (uint32_t) diff | (1 << 31)
+		? (uint32_t) diff | (((uint32_t ) 1u) << 31)
 		: (uint32_t) diff;
 }