plugins: minor, rename local functions

Prevent name clash with upcoming refactor and any confusion with
src/plugin.c functions with the same name.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

plugins/usr.c

@@ -108,7 +108,7 @@ static plugin_t plugin = {
 	.depends = { "bootmisc", },
 };
 
-PLUGIN_INIT(plugin_init)
+PLUGIN_INIT(__init)
 {
 	int fd;
 
@@ -123,7 +123,7 @@ PLUGIN_INIT(plugin_init)
 	plugin_register(&plugin);
 }
 
-PLUGIN_EXIT(plugin_exit)
+PLUGIN_EXIT(__exit)
 {
 	plugin_unregister(&plugin);
 	iwatch_exit(&iw_usr);