Switch implicit rules into errors

There are no users of these, other than the PDK, which is already
exempted.

Test: m nothing
Test: Check for warnings in downstream build_test logs
Test: Check for warnings in last two days of continuous build logs
Change-Id: I7c7d0068e928e591355c39d22635af86b8f43f1b

ui/build/kati.go

@@ -85,7 +85,7 @@ func runKati(ctx Context, config Config) {
 
 	// PDK builds still uses a few implicit rules
 	if !config.IsPdkBuild() {
-		args = append(args, "--warn_implicit_rules")
+		args = append(args, "--werror_implicit_rules")
 	}
 
 	if !config.BuildBrokenDupRules() {