[kotlin] Ensure .targets file gets put into NuGet package. (dotnet#404)

Author: jpobst

Directory.Build.props

@@ -16,4 +16,12 @@
     <!-- .NET 6+ packages support back to API-21 -->
     <SupportedOSPlatformVersion>21</SupportedOSPlatformVersion>
   </PropertyGroup>
+  
+  <!-- Folders that .targets files need to go into -->
+  <ItemGroup>
+    <AndroidXNuGetTargetFolders Include="build\monoandroid90" />
+    <AndroidXNuGetTargetFolders Include="build\net6.0-android31.0" />
+    <AndroidXNuGetTargetFolders Include="buildTransitive\monoandroid90" />
+    <AndroidXNuGetTargetFolders Include="buildTransitive\net6.0-android31.0" />
+  </ItemGroup>
 </Project>