staging: dgap: fix missing header inclusion

Include slab.h to fix the following compilation error:

drivers/staging/dgap/dgap_fep5.c: In function ‘dgap_do_config_load’:
drivers/staging/dgap/dgap_fep5.c:78:2: error: implicit declaration of function ‘kzalloc’ [-Werror=implicit-function-declaration]

Signed-off-by: Vincent Stehlé <[e-mail]>
Cc: Lidza Louina <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Scott H Kilau <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/dgap/dgap_fep5.c

···
 #include <asm/uaccess.h>	/* For copy_from_user/copy_to_user */
 #include <linux/tty.h>
 #include <linux/tty_flip.h>	/* For tty_schedule_flip */
+#include <linux/slab.h>
 
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,39)
 #include <linux/sched.h>