archive_plugin: pass const string to method open()

src/archive/bz2_archive_plugin.c

@@ -96,7 +96,7 @@ bz2_destroy(struct bz2_archive_file *data)
 /* archive open && listing routine */
 
 static struct archive_file *
-bz2_open(char *pathname)
+bz2_open(const char *pathname)
 {
 	struct bz2_archive_file *context;
 	int len;

src/archive/iso9660_archive_plugin.c

@@ -88,7 +88,7 @@ listdir_recur(const char *psz_path, struct iso9660_archive_file *context)
 }
 
 static struct archive_file *
-iso9660_archive_open(char * pathname)
+iso9660_archive_open(const char *pathname)
 {
 	struct iso9660_archive_file *context =
 		g_new(struct iso9660_archive_file, 1);

src/archive/zzip_archive_plugin.c

@@ -50,7 +50,7 @@ zzip_quark(void)
 /* archive open && listing routine */
 
 static struct archive_file *
-zzip_archive_open(char *pathname)
+zzip_archive_open(const char *pathname)
 {
 	struct zzip_archive *context = g_malloc(sizeof(*context));
 	ZZIP_DIRENT dirent;