Set dir_third_party_nanopb in .gn

Configures dir_third_party_nanopb in the .gn file so toolchains don't
need to set it.

Change-Id: I171e80693e9c88f760dd9b6817764c9b402e41a9
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/33800
Reviewed-by: Rob Mohr <mohrr@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Armando Montanez <amontanez@google.com>
diff --git a/.gn b/.gn
index 60ce6bd..e802532 100644
--- a/.gn
+++ b/.gn
@@ -13,3 +13,8 @@
 # the License.
 
 buildconfig = "//BUILDCONFIG.gn"
+
+default_args = {
+  # Set the nanopb directory for protobuf generation.
+  dir_pw_third_party_nanopb = "//third_party/nanopb"
+}