Exclude generated_plugin_registrant.cc (#3198)

diff --git a/.gitignore b/.gitignore
index 88ce490..d756050 100644
--- a/.gitignore
+++ b/.gitignore
@@ -37,6 +37,7 @@
 generated_plugin_registrant.dart
 GeneratedPluginRegistrant.h
 GeneratedPluginRegistrant.m
+generated_plugin_registrant.cc
 GeneratedPluginRegistrant.java
 GeneratedPluginRegistrant.swift
 build/