Ignore disabled modules when generating ndk sysroot

When linux_bionic is turned on, there are two host modules for the
preprocessed ndk headers, but only one is enabled. So exit early for the
disabled one.

Bug: 31559095
Test: Diff out/soong/build.ninja before/after, no change.
Test: Turn on linux_bionic, no longer see a panic.
Change-Id: I204a884a83ccf21c96088e97c19a0cffe029ec99

cc/ndk_sysroot.go

@@ -90,6 +90,10 @@ func (n *ndkSingleton) GenerateBuildActions(ctx blueprint.SingletonContext) {
 	installPaths := []string{}
 	licensePaths := []string{}
 	ctx.VisitAllModules(func(module blueprint.Module) {
+		if m, ok := module.(android.Module); ok && !m.Enabled() {
+			return
+		}
+
 		if m, ok := module.(*headerModule); ok {
 			installPaths = append(installPaths, m.installPaths...)
 			licensePaths = append(licensePaths, m.licensePath.String())