MAINTAINERS: add myself as reviewer for SquashFS

As I have followed the development of the SquashFS support in U-Boot
as part of Joao Marcos work, it makes sense to get Cc'ed on
contributions/bug reports related to the squashfs support.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 87603d4..fdbfd2b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -983,6 +983,7 @@
 
 SQUASHFS
 M:	Joao Marcos Costa <joaomarcos.costa@bootlin.com>
+R:	Thomas Petazzoni <thomas.petazzoni@bootlin.com>
 S:	Maintained
 F:	fs/squashfs/
 F:	include/sqfs.h