Unify include statements for protobuf

Always use <system> rather than "user" style includes for Protobuf
headers. This removes a special case from the amalgamated build
generator.

Change-Id: I1d253cf5ff6c31f98b675d19c679f5cd9305d533
diff --git a/src/ipc/deferred.cc b/src/ipc/deferred.cc
index 5542cd5..d923ffc 100644
--- a/src/ipc/deferred.cc
+++ b/src/ipc/deferred.cc
@@ -16,7 +16,7 @@
 
 #include "perfetto/ipc/deferred.h"
 
-#include "google/protobuf/message_lite.h"
+#include <google/protobuf/message_lite.h>
 #include "perfetto/base/logging.h"
 
 namespace perfetto {