fix: fix getting shadow root when component is functional (#1560)

Author: zrh122

Diff for: lib/runtime/componentNormalizer.js

@@ -62,7 +62,12 @@ export default function normalizeComponent (
     options._ssrRegister = hook
   } else if (injectStyles) {
     hook = shadowMode
-      ? function () { injectStyles.call(this, this.$root.$options.shadowRoot) }
+      ? function () {
+        injectStyles.call(
+          this,
+          (options.functional ? this.parent : this).$root.$options.shadowRoot
+        )
+      }
       : injectStyles
   }