pw_kvs: Move span traits outside of public header

Change-Id: I1cabd7b8121cf227189d8aa4d86e8a036866c7ed
diff --git a/pw_kvs/BUILD b/pw_kvs/BUILD
index 0bb2120..01b63ea 100644
--- a/pw_kvs/BUILD
+++ b/pw_kvs/BUILD
@@ -34,6 +34,7 @@
         "public/pw_kvs/internal/hash.h",
         "public/pw_kvs/internal/key_descriptor.h",
         "public/pw_kvs/internal/sector_descriptor.h",
+        "public/pw_kvs/internal/span_traits.h",
         "pw_kvs_private/macros.h",
     ],
     hdrs = [