MAINTAINERS: add IIO GTS tests

Add undersigned as a maintainer for IIO GTS helper's KUnit tests.
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/52c66fe2798192529738ac2ab98a27230a6ad8cd.1705328293.git.mazziesaccount@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

MAINTAINERS

@@ -10405,6 +10405,7 @@ L:	linux-iio@vger.kernel.org
 S:	Maintained
 F:	drivers/iio/industrialio-gts-helper.c
 F:	include/linux/iio/iio-gts-helper.h
+F:	drivers/iio/test/iio-test-gts.c
 
 IIO MULTIPLEXER
 M:	Peter Rosin <peda@axentia.se>