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

Test: adb host-features | grep push_sync
Bug: None
Change-Id: Ia7a7e9c3c2e6632a9c954fbad8f64602a21d9757
diff --git a/adb/transport.h b/adb/transport.h
index 7304404..57fc988 100644
--- a/adb/transport.h
+++ b/adb/transport.h
@@ -51,6 +51,8 @@
 extern const char* const kFeatureStat2;
 // The server is running with libusb enabled.
 extern const char* const kFeatureLibusb;
+// The server supports `push --sync`.
+extern const char* const kFeaturePushSync;
 
 class atransport {
 public: