src/drivers/usb/gadget.c: Add missing include

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

Change-Id: I1ca111a30153526c3915bf393208fb2b4c569c01
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43347
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/usb/gadget.c

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-#include <stddef.h>
+#include <stdint.h>
 #include <console/console.h>
 #include <string.h>