Only show deprecation when using next/css (#762)

Author: timneutkens

lib/css.js

@@ -1,5 +1,7 @@
 import { deprecated } from './utils'
 const css = require('glamor')
+// Needed because of the mutation below
+delete require.cache[require.resolve('glamor')]
 
 for (const [k, v] of Object.entries(css)) {
   if (typeof v === 'function') {