DEVELOPERS: add Theo Debrouwere as contact for beagleboardx15_defconfig

Theo added this defconfig in 2016, and is listed as the contact for
the pugixml package, so it probably makes sense to have him listed as
a contact for the beagleboardx15_defconfig as well.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Theo Debrouwere <t.debrouwere@televic.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>