drivers: dgnc: Include appropriate header file in dgnc_trace.c

Include appropriate header file dgnc/dgnc_trace.h in dgnc_trace.c
because function dgnc_tracer_free() has its prototype declaration in the
header file.

This eliminates the following warning in dgnc_trace.c:
drivers/staging/dgnc/dgnc_trace.c:180:6: warning: no previous prototype for ‘dgnc_tracer_free’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>