pw_presubmit: gn check system and generated files

- In presubmit, do gn check in a separate step and specify
  --check-generated and --check-system.
- Fix gn check issues in pw_fuzzer by adding a missing dependency.
- Disable include checking for generated pw_rpc headers since they
  include internal pw_rpc headers.

Change-Id: I8b9f03957ac60166eeb141bcba9dc493bfc6350c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44562
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_protobuf_compiler/proto.gni b/pw_protobuf_compiler/proto.gni
index 51d443f..33fe5d7 100644
--- a/pw_protobuf_compiler/proto.gni
+++ b/pw_protobuf_compiler/proto.gni
@@ -212,6 +212,7 @@
       public_deps = [ "$dir_pw_third_party/nanopb" ] + invoker.deps
       sources = invoker.outputs
       public = filter_include(sources, [ "*.pb.h" ])
+      check_includes = false
     }
   }
 }
@@ -244,6 +245,7 @@
                     "$dir_pw_rpc/raw:method_union",
                   ] + invoker.deps
     public = invoker.outputs
+    check_includes = false
   }
 }