Add host feature for `push --sync` support.

Test: adb host-features | grep push_sync
Bug: None
Change-Id: Ia7a7e9c3c2e6632a9c954fbad8f64602a21d9757
diff --git a/adb.cpp b/adb.cpp
index 808d8ff..a7706a0 100644
--- a/adb.cpp
+++ b/adb.cpp
@@ -1140,6 +1140,7 @@
         if (should_use_libusb()) {
             features.insert(kFeatureLibusb);
         }
+        features.insert(kFeaturePushSync);
         SendOkay(reply_fd, FeatureSetToString(features));
         return 0;
     }