Fix inputs for main_dex_list action.

BUG=

Review-Url: https://codereview.chromium.org/2500113003
Cr-Original-Commit-Position: refs/heads/master@{#432007}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 237ee6948e7127d4d8e2022a63d26410d1080378

config/android/internal_rules.gni

@@ -912,6 +912,11 @@ if (enable_java_templates) {
 
         main_dex_rules = "//build/android/main_dex_classes.flags"
 
+        if (!defined(inputs)) {
+          inputs = []
+        }
+        inputs += [ main_dex_rules ]
+
         outputs = [
           _main_dex_list_path,
         ]