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>

drivers/media/common/videobuf2/Makefile

@@ -1,11 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0
+videobuf2-common-objs := videobuf2-core.o
 
-obj-$(CONFIG_VIDEOBUF2_CORE) += videobuf2-core.o
+ifeq ($(CONFIG_TRACEPOINTS),y)
+  videobuf2-common-objs += vb2-trace.o
+endif
+
+obj-$(CONFIG_VIDEOBUF2_CORE) += videobuf2-common.o
 obj-$(CONFIG_VIDEOBUF2_V4L2) += videobuf2-v4l2.o
 obj-$(CONFIG_VIDEOBUF2_MEMOPS) += videobuf2-memops.o
 obj-$(CONFIG_VIDEOBUF2_VMALLOC) += videobuf2-vmalloc.o
 obj-$(CONFIG_VIDEOBUF2_DMA_CONTIG) += videobuf2-dma-contig.o
 obj-$(CONFIG_VIDEOBUF2_DMA_SG) += videobuf2-dma-sg.o
 obj-$(CONFIG_VIDEOBUF2_DVB) += videobuf2-dvb.o
-ifeq ($(CONFIG_TRACEPOINTS),y)
-  obj-$(CONFIG_VIDEOBUF2_CORE) += vb2-trace.o
-endif