add primer-dropdown to primer-css

Author: shawnbot

modules/primer-css/package.json

@@ -36,6 +36,7 @@
     "primer-buttons": "2.4.0",
     "primer-cards": "0.5.0",
     "primer-core": "6.4.0",
+    "primer-dropdown": "0.1.0",
     "primer-forms": "1.4.0",
     "primer-labels": "1.5.0",
     "primer-layout": "1.4.0",