Add type casts to silence warnings intorduced by protobuf v3.0.0

Bug: b/28974522
Change-Id: I1a5a26ea503747b786ed38ae5d6a586340a5fe2e
diff --git a/payload_generator/extent_utils.cc b/payload_generator/extent_utils.cc
index 72e4b7c..89ccca2 100644
--- a/payload_generator/extent_utils.cc
+++ b/payload_generator/extent_utils.cc
@@ -99,8 +99,9 @@
 string ExtentsToString(const vector<Extent>& extents) {
   string ext_str;
   for (const Extent& e : extents)
-    ext_str += base::StringPrintf(
-        "[%" PRIu64 ", %" PRIu64 "] ", e.start_block(), e.num_blocks());
+    ext_str += base::StringPrintf("[%" PRIu64 ", %" PRIu64 "] ",
+                                  static_cast<uint64_t>(e.start_block()),
+                                  static_cast<uint64_t>(e.num_blocks()));
   return ext_str;
 }