MAINTAINERS: add missing file in ext4 entry

include/trace/events/ext4.h belongs to ext4 module, add the file path into
ext4 entry in MAINTAINERS.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20201030022435.1136-1-yuchao0@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>