add generated_plugins.cmake (#116581)

Added files to the .gitignore that are generated on each "flutter pub get", so it's useless to ever commit these to a git repository.

.gitignore

@@ -110,10 +110,12 @@ unlinked_spec.ds
 # Windows
 **/windows/flutter/generated_plugin_registrant.cc
 **/windows/flutter/generated_plugin_registrant.h
+**/windows/flutter/generated_plugins.cmake
 
 # Linux
 **/linux/flutter/generated_plugin_registrant.cc
 **/linux/flutter/generated_plugin_registrant.h
+**/linux/flutter/generated_plugins.cmake
 
 # Coverage
 coverage/