Turn implicit rules into warnings

Test: build_test in downstream branches
Change-Id: I853e3fa11793cc76ad568b74f7dc31c3d1c5582b
diff --git a/ui/build/kati.go b/ui/build/kati.go
index 743da4a..1494688 100644
--- a/ui/build/kati.go
+++ b/ui/build/kati.go
@@ -83,6 +83,11 @@
 		"-f", "build/make/core/main.mk",
 	}
 
+	// PDK builds still uses a few implicit rules
+	if !config.IsPdkBuild() {
+		args = append(args, "--warn_implicit_rules")
+	}
+
 	if !config.BuildBrokenDupRules() {
 		args = append(args, "--werror_overriding_commands")
 	}