[media] solo6x10: fix broken Makefile

The Makefile used := instead of +=, this broke the compatibility build.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/staging/media/solo6x10/Makefile

 solo6x10-y := core.o i2c.o p2m.o v4l2.o tw28.o gpio.o disp.o enc.o v4l2-enc.o g723.o
 
-obj-$(CONFIG_SOLO6X10) := solo6x10.o
+obj-$(CONFIG_SOLO6X10) += solo6x10.o