Make ndk.timestamp optional

Singleton rules need to manually specify Optional: true, or they will be
built for any ninja invocation that has no targets specified.

Test: m -j on tradefed build
Change-Id: Ifc060b25bc32e664c48731eb8a0d963b73574591

cc/ndk_sysroot.go

@@ -108,5 +108,6 @@ func (n *ndkSingleton) GenerateBuildActions(ctx blueprint.SingletonContext) {
 		Rule:      android.Touch,
 		Outputs:   []string{getNdkSysrootTimestampFile(ctx).String()},
 		Implicits: installPaths,
+		Optional:  true,
 	})
 }