pw_polyfill: add missing dependency

Change-Id: I72d8dab18a0ad61763adc01d5093b65427956ee8
diff --git a/pw_polyfill/BUILD b/pw_polyfill/BUILD
index 3e77f6d..12fdbd1 100644
--- a/pw_polyfill/BUILD
+++ b/pw_polyfill/BUILD
@@ -63,6 +63,7 @@
     ],
     deps = [
         ":pw_polyfill",
+        ":standard_library",
         "//pw_unit_test",
     ],
 )