[media] v4l2: move tracepoint generation into separate file

To compile videobuf2-core as a module, the vb2_* tracepoints must be
exported from the videodev module. Instead of exporting vb2 tracepoint
symbols from v4l2-ioctl.c, move the tracepoint generation into a separate
file. This patch fixes the following build error in the modpost stage,
introduced by 2091f518 ("[media] videobuf2: add trace events"):

    ERROR: "__tracepoint_vb2_buf_done" undefined!
    ERROR: "__tracepoint_vb2_dqbuf" undefined!
    ERROR: "__tracepoint_vb2_qbuf" undefined!
    ERROR: "__tracepoint_vb2_buf_queue" undefined!

Suggested-by: Steven Rostedt <rostedt@goodmis.org>

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>