[jnigen] Add proguard-rules for kotlin_plugin (https://github.com/dart-lang/jnigen/issues/296)

pkgs/jnigen/example/kotlin_plugin/example/android/app/proguard-rules.pro

diff --git a/pkgs/jnigen/example/kotlin_plugin/example/android/app/proguard-rules.pro b/pkgs/jnigen/example/kotlin_plugin/example/android/app/proguard-rules.pro
new file mode 100644
index 0000000..8c1218b
--- /dev/null
+++ b/pkgs/jnigen/example/kotlin_plugin/example/android/app/proguard-rules.pro
@@ -0,0 +1 @@
+-keep class kotlin.coroutines.** { *; }