chan_vpb: Fix 'catching polymorphic type ... by value' error

Fixes the following compile error:

    chan_vpb.cc:2688:26: error: catching polymorphic type
        ‘class std::exception’ by value

Change-Id: Ic87bc357d72427d77626735c83200fd278a7a649

channels/chan_vpb.cc

@@ -2685,7 +2685,7 @@ static enum ast_module_load_result load_module()
 	ast_format_cap_append(vpb_tech_indicate.capabilities, ast_format_slin, 0);
 	try {
 		num_cards = vpb_get_num_cards();
-	} catch (std::exception e) {
+	} catch (std::exception&) {
 		ast_log(LOG_ERROR, "No Voicetronix cards detected\n");
 		return AST_MODULE_LOAD_DECLINE;
 	}