Add missing format_test dep.

Change-Id: If5107700a9c8dab0c8f617fb2bd23741c3dfe080
diff --git a/pw_string/BUILD b/pw_string/BUILD
index 22e198b..8959ea1 100644
--- a/pw_string/BUILD
+++ b/pw_string/BUILD
@@ -43,7 +43,10 @@
 pw_cc_test(
     name = "format_test",
     srcs = ["format_test.cc"],
-    deps = ["//pw_string"],
+    deps = [
+        "//pw_span",
+        "//pw_string",
+    ],
 )
 
 pw_cc_test(