uapi/linux/stddef.h: Add include guards

Add include guard wrapper define to uapi/linux/stddef.h to prevent macro
redefinition errors when stddef.h is included more than once. This was not
needed before since the only contents already used a redefinition test.
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Link: https://lore.kernel.org/r/20220329171252.57279-1-tadeusz.struk@linaro.org
Fixes: 50d7bd38 ("stddef: Introduce struct_group() helper macro")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

include/uapi/linux/stddef.h

 /* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_STDDEF_H
+#define _UAPI_LINUX_STDDEF_H
+
 #include <linux/compiler_types.h>
 
 #ifndef __always_inline
@@ -41,3 +44,4 @@
 		struct { } __empty_ ## NAME; \
 		TYPE NAME[]; \
 	}
+#endif