Add class template deduction guide.

This is needed by some build systems.

Change-Id: I992ccc045cbc72da1c49bde8368512a3cb102d6b
diff --git a/pw_protobuf/public/pw_protobuf/encoder.h b/pw_protobuf/public/pw_protobuf/encoder.h
index 89c4e78..360b88f 100644
--- a/pw_protobuf/public/pw_protobuf/encoder.h
+++ b/pw_protobuf/public/pw_protobuf/encoder.h
@@ -364,4 +364,7 @@
   std::array<size_t*, kMaxNestedDepth> stack_;
 };
 
+// Explicit template argument deduction to hide warnings.
+NestedEncoder()->NestedEncoder<>;
+
 }  // namespace pw::protobuf