media: vb2: Makefile: place vb2-trace together with vb2-core

We don't want a separate module for vb2-trace.

That fixes this warning:

	WARNING: modpost: missing MODULE_LICENSE() in drivers/media/common/videobuf2/vb2-trace.o

When building as module.

While here, add a SPDX header.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>