drm/amd/display/dc/gpio/diagnostics/hw_factory_diag: Include our own header containing prototypes

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/gpio/diagnostics/hw_factory_diag.c:50:6: warning: no previous prototype for ‘dal_hw_factory_diag_fpga_init’ [-Wmissing-prototypes]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/gpio/diagnostics/hw_factory_diag.c

@@ -28,6 +28,7 @@
  */
 
 #include "dm_services.h"
+#include "hw_factory_diag.h"
 #include "include/gpio_types.h"
 #include "../hw_factory.h"