Add required reference

Author: JunTaoLuo

src/Microsoft.Extensions.Configuration.AzureKeyVault/Microsoft.Extensions.Configuration.AzureKeyVault.csproj

@@ -14,6 +14,7 @@
     <ProjectReference Include="..\Microsoft.Extensions.Configuration.FileExtensions\Microsoft.Extensions.Configuration.FileExtensions.csproj" />
     <PackageReference Include="Microsoft.IdentityModel.Clients.ActiveDirectory" Version="$(IdentityModelActiveDirectoryVersion)" />
     <PackageReference Include="Microsoft.Azure.KeyVault" Version="$(AzureKeyVaultVersion)" />
+    <PackageReference Include="System.Net.Http" Version="$(CoreFxVersion)" />
   </ItemGroup>
 
 </Project>