initrd: Add the preprocessor guard in initrd.h

Add the preprocessor guard in initrd.h to prevent possible
build error from the multiple inclusion of same header file
multiple time.
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

include/linux/initrd.h

 /* SPDX-License-Identifier: GPL-2.0 */
 
+#ifndef __LINUX_INITRD_H
+#define __LINUX_INITRD_H
+
 #define INITRD_MINOR 250 /* shouldn't collide with /dev/ram* too soon ... */
 
 /* starting block # of image */
@@ -24,3 +27,5 @@ extern char __initramfs_start[];
 extern unsigned long __initramfs_size;
 
 void console_on_rootfs(void);
+
+#endif /* __LINUX_INITRD_H */