fix: Import @DoNotStrip (#770)

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### Summary

- Fixes #768

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packages/create-react-native-library/templates/nitro-module/android/src/main/java/com/margelo/nitro/{%- project.package_dir %}/{%- project.name %}.kt

@@ -1,4 +1,6 @@
 package com.margelo.nitro.<%- project.package %>
+
+import com.facebook.proguard.annotations.DoNotStrip
 
 @DoNotStrip
 class <%- project.name %> : Hybrid<%- project.name %>Spec() {