usb: typec: altmodes: displayport: Supply missing displayport.h include file

If the header file containing a function's prototype isn't included by
the sourcefile containing the associated function, the build system
complains of missing prototypes.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/typec/altmodes/displayport.c:511:5: warning: no previous prototype for ‘dp_altmode_probe’ [-Wmissing-prototypes]
 drivers/usb/typec/altmodes/displayport.c:551:6: warning: no previous prototype for ‘dp_altmode_remove’ [-Wmissing-prototypes]

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200703174148.2749969-22-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/typec/altmodes/displayport.c

@@ -13,6 +13,7 @@
 #include <linux/module.h>
 #include <linux/usb/pd_vdo.h>
 #include <linux/usb/typec_dp.h>
+#include "displayport.h"
 
 #define DP_HEADER(_dp, cmd)		(VDO((_dp)->alt->svid, 1, cmd) | \
 					 VDO_OPOS(USB_TYPEC_DP_MODE))