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>

drivers/staging/dgap/dgap_trace.c

@@ -37,6 +37,7 @@
 #include <linux/vmalloc.h>
 
 #include "dgap_driver.h"
+#include "dgap_trace.h"
 
 #define TRC_TO_CONSOLE 1