staging: dgap: Add missing #include <linux/slab.h>

drivers/staging/dgap/dgap_driver.c: In function ‘dgap_cleanup_module’:
drivers/staging/dgap/dgap_driver.c:423: error: implicit declaration of function ‘kfree’
drivers/staging/dgap/dgap_driver.c: In function ‘dgap_driver_kzmalloc’:
drivers/staging/dgap/dgap_driver.c:940: error: implicit declaration of function ‘kmalloc’
drivers/staging/dgap/dgap_driver.c:940: warning: initialization makes pointer from integer without a cast

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>