ArchivePlugin: rename struct archive_plugin to ArchivePlugin

src/archive/ArchivePlugin.cxx

@@ -25,7 +25,7 @@
 #include <assert.h>
 
 ArchiveFile *
-archive_file_open(const struct archive_plugin *plugin, const char *path,
+archive_file_open(const ArchivePlugin *plugin, const char *path,
 		  Error &error)
 {
 	assert(plugin != nullptr);