Add a missing include for 0_2_1 version. (#51)

Signed-off-by: Gregory Brail <[email protected]>

Author: gbrail

include/proxy-wasm/null_plugin.h

@@ -38,6 +38,7 @@ namespace proxy_wasm {
 struct NullPluginRegistry {
   void (*proxy_abi_version_0_1_0_)() = nullptr;
   void (*proxy_abi_version_0_2_0_)() = nullptr;
+  void (*proxy_abi_version_0_2_1_)() = nullptr;
   void (*proxy_on_log_)(uint32_t context_id) = nullptr;
   uint32_t (*proxy_validate_configuration_)(uint32_t root_context_id,
                                             uint32_t plugin_configuration_size) = nullptr;