Add support for extensions_unittests to the GN build

third_party/protobuf/BUILD.gn

Index: third_party/protobuf/BUILD.gn
diff --git a/third_party/protobuf/BUILD.gn b/third_party/protobuf/BUILD.gn
index 973672b94f7acdd2aa7c4b363d3b2afa53e5dbc0..1367d8a16b7fc8df45be4d1f0c09ad6886f8c713 100644
--- a/third_party/protobuf/BUILD.gn
+++ b/third_party/protobuf/BUILD.gn
@@ -30,10 +30,13 @@ if (component_mode == "shared_library") {
   }
 }
 
-# This condif should be applied to targets using generated code from the proto
+# This config should be applied to targets using generated code from the proto
 # compiler. It sets up the include directories properly.
 config("using_proto") {
-  include_dirs = [ "$root_gen_dir/protoc_out" ]
+  include_dirs = [
+    "src",
+    "$root_gen_dir/protoc_out",
+  ]
 }
 
 protobuf_lite_sources = [