releasetools: add dep to checkvintf

'required' only works on python_binary_host.
Test: modify checkvintf source code, build ota_from_target_files

Change-Id: Ieac68b39b1ccabae679a1679f184cb3aeaaeefb6

tools/releasetools/Android.bp

@@ -105,6 +105,7 @@ python_defaults {
     ],
     required: [
         "brillo_update_payload",
+        "checkvintf",
     ],
 }
 
@@ -343,6 +344,9 @@ python_binary_host {
         "releasetools_img_from_target_files",
         "releasetools_ota_from_target_files",
     ],
+    required: [
+        "checkvintf",
+    ],
 }
 
 python_binary_host {