drivers: dgap: Include appropriate header file in dgap_trace.c

drivers: dgap: Include appropriate header file in dgap_trace.c

Include appropriate header file dgap/dgap_trace.h in dgap_trace.c
because function dgap_tracer_free() have its prototype declaration in
the header file.

This eliminates the following warning in dgap_trace.c:
drivers/staging/dgap/dgap_trace.c:181:6: warning: no previous prototype for ‘dgap_tracer_free’ [-Wmissing-prototypes]

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