fix issue with missing sample_main in sample plugin

Change-Id: Ia17511e3997cdcf1d0991e62e8e2d3fb8812d133
Signed-off-by: Damjan Marion <damarion@cisco.com>

src/examples/sample-plugin/sample/sample.c

@@ -63,6 +63,8 @@ VLIB_PLUGIN_REGISTER () = {
 };
 /* *INDENT-ON* */
 
+sample_main_t sample_main;
+
 /**
  * @brief Enable/disable the macswap plugin. 
  *