There was a trouble with vb2-trace: instead of being part of
VB2 core, it was stored at V4L2 videodev. That was wrong,
as it doesn't actually belong to V4L2 core.

Now that vb2 is not part of v4l2-core, its trace functions
should be moved altogether. So, move it to its rightful
place: at videobuf2-core.

That fixes those errors:
	drivers/media/common/videobuf2/videobuf2-core.o: In function `__read_once_size':
	./include/linux/compiler.h:183: undefined reference to `__tracepoint_vb2_buf_queue'
	./include/linux/compiler.h:183: undefined reference to `__tracepoint_vb2_buf_queue'
	./include/linux/compiler.h:183: undefined reference to `__tracepoint_vb2_buf_done'
	./include/linux/compiler.h:183: undefined reference to `__tracepoint_vb2_buf_done'
	./include/linux/compiler.h:183: undefined reference to `__tracepoint_vb2_qbuf'
	./include/linux/compiler.h:183: undefined reference to `__tracepoint_vb2_qbuf'
	./include/linux/compiler.h:183: undefined reference to `__tracepoint_vb2_dqbuf'
	./include/linux/compiler.h:183: undefined reference to `__tracepoint_vb2_dqbuf'
	drivers/media/common/videobuf2/videobuf2-core.o:(__jump_table+0x10): undefined reference to `__tracepoint_vb2_buf_queue'
	drivers/media/common/videobuf2/videobuf2-core.o:(__jump_table+0x28): undefined reference to `__tracepoint_vb2_buf_done'
	drivers/media/common/videobuf2/videobuf2-core.o:(__jump_table+0x40): undefined reference to `__tracepoint_vb2_qbuf'
	drivers/media/common/videobuf2/videobuf2-core.o:(__jump_table+0x58): undefined reference to `__tracepoint_vb2_dqbuf'
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>