pw_kvs: Support multiple entry formats

Change-Id: If25ea00210178b8495585a651d3df6ad8347ee82
diff --git a/pw_kvs/BUILD b/pw_kvs/BUILD
index 571e44f..218d199 100644
--- a/pw_kvs/BUILD
+++ b/pw_kvs/BUILD
@@ -29,6 +29,7 @@
         "checksum.cc",
         "entry.cc",
         "flash_memory.cc",
+        "format.cc",
         "key_value_store.cc",
         "public/pw_kvs/internal/entry.h",
         "public/pw_kvs/internal/hash.h",