MAINTAINERS/ACCEL: Add include/drm/drm_accel.h to the accel entry

get_maintainer.pl does not suggest Oded Gabbay, the DRM COMPUTE
ACCELERATORS DRIVERS AND FRAMEWORK maintainer for changes that touch
the Accel Subsystem header - drm_accel.h.  This is because that file is
missing from the Accel Subsystem entry.  Fix this.

Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>