.gn: add dir_pw_third_party_nanopb

Change-Id: I06fe2c9580a48d155c773ee7085121035342a44d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/36024
Reviewed-by: Anthony DiGirolamo <tonymd@google.com>
Commit-Queue: Anthony DiGirolamo <tonymd@google.com>
diff --git a/.gn b/.gn
index 9e68813..b00e806 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"
+}