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.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

MAINTAINERS

@@ -6614,6 +6614,7 @@ Q:	http://patchwork.ozlabs.org/project/linux-ext4/list/
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git
 F:	Documentation/filesystems/ext4/
 F:	fs/ext4/
+F:	include/trace/events/ext4.h
 
 Extended Verification Module (EVM)
 M:	Mimi Zohar <zohar@linux.ibm.com>