Btrfs: extent map selftest: add missing void parameter to btrfs_test_extent_map

Add a missing void parameter to function btrfs_test_extent_map, fixes
sparse warning:

warning: non-ANSI function declaration of function 'btrfs_test_extent_map'

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David Sterba <dsterba@suse.com>