pw_rpc: Add Bazel packet proto library

This change defines the packet proto as a Bazel proto library for use
from Bazel builds.

Change-Id: I6a4c70ddfad0d0ef591f3f6aa7a6ee880ed8352a
diff --git a/pw_rpc/BUILD b/pw_rpc/BUILD
index 9c03beb..fdd6f81 100644
--- a/pw_rpc/BUILD
+++ b/pw_rpc/BUILD
@@ -82,6 +82,14 @@
     ],
 )
 
+proto_library(
+    name = "packet_proto",
+    srcs = [
+        "pw_rpc_protos/packet.proto",
+    ]
+)
+
+
 pw_cc_test(
     name = "server_test",
     srcs = [