fix compile warnings for unused code

[heitbaum]

../mtpfs.c: In function 'lookup_folder_id':
../mtpfs.c:477:27: warning: unused variable 'storage' [-Wunused-variable]
477 | LIBMTP_devicestorage_t *storage;
| ^~~~~~~
../mtpfs.c: In function 'mtpfs_readdir':
../mtpfs.c:860:18: warning: unused variable 'name' [-Wunused-variable]
860 | gchar *name;
| ^~~~
../mtpfs.c:966:25: warning: variable 'tmp' set but not used [-Wunused-but-set-variable]
966 | LIBMTP_file_t *file, *tmp;
| ^~~
../mtpfs.c:914:7: warning: unused variable 'i' [-Wunused-variable]
914 | int i;
| ^
../mtpfs.c: In function 'mtpfs_rename':
../mtpfs.c:1514:23: warning: variable 'parent_id' set but not used [-Wunused-but-set-variable]
1514 | int folder_id = -1, parent_id;
| ^~~~~~~~~
../mtpfs.c: In function 'mtpfs_init':
../mtpfs.c:1616:27: warning: unused variable 'storage' [-Wunused-variable]
1616 | LIBMTP_devicestorage_t *storage;
| ^~~~~~~
../mtpfs.c: In function 'main':
../mtpfs.c:1658:7: warning: unused variable 'opt' [-Wunused-variable]
1658 | int opt;
| ^~~
../mtpfs.c: At top level:
../mtpfs.c:78:1: warning: 'check_lost_files' defined but not used [-Wunused-function]
78 | check_lost_files ()
| ^~~~~~~~~~~~~~~~