pw_protobuf_compiler: Add missing import

This change imports nanopb.gni into proto.gni for the nanopb build arg.

Change-Id: I62ad4837e296d66be3c14d549bfa8f111c999c85
diff --git a/pw_protobuf_compiler/proto.gni b/pw_protobuf_compiler/proto.gni
index 48336f6..13e2e67 100644
--- a/pw_protobuf_compiler/proto.gni
+++ b/pw_protobuf_compiler/proto.gni
@@ -18,6 +18,7 @@
 import("$dir_pw_build/input_group.gni")
 import("$dir_pw_build/python_script.gni")
 import("$dir_pw_build/target_types.gni")
+import("nanopb.gni")
 declare_args() {
   # Generators with which to compile protobuf code. These are used by the
   # pw_proto_library template to determine which build targets to create.