drivers: gpu: Include appropriate header file in drm_usb.c

Include appropriate header file include/drm/drm_usb.h in drm/drm_usb.c
because functions drm_get_usb_dev(), drm_usb_init() and drm_usb_exit()
have their prototype declarations in the header file.

This eliminates the following warning in drm/drm_usb.c:
drivers/gpu/drm/drm_usb.c:5:5: warning: no previous prototype for
‘drm_get_usb_dev’ [-Wmissing-prototypes]
drivers/gpu/drm/drm_usb.c:61:5: warning: no previous prototype for
‘drm_usb_init’ [-Wmissing-prototypes]
drivers/gpu/drm/drm_usb.c:75:6: warning: no previous prototype for
‘drm_usb_exit’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>