soc/rockchip/rk3399/display.c: Add missing include

Replace unused <stddef.h> with missing <stdint.h>.

Change-Id: Ibdde8fb5ec5bf7d25facd78064a7837d24fa2c8a
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43342
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/rockchip/rk3399/display.c

@@ -7,7 +7,7 @@
 #include <delay.h>
 #include <edid.h>
 #include <gpio.h>
-#include <stddef.h>
+#include <stdint.h>
 #include <soc/addressmap.h>
 #include <soc/clock.h>
 #include <soc/display.h>