libfdt: Update the inclusion define protection

Inspired by commit 9130ba8 ("scripts/dtc: Update to upstream
version v1.4.6-9-gaadd0b65c987")

Signed-off-by: Cristian Birsan <cristian.birsan@microchip.com>

Author: cristibirsan

include/linux/libfdt_env.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0 */
-#ifndef _LIBFDT_ENV_H
-#define _LIBFDT_ENV_H
+#ifndef LIBFDT_ENV_H
+#define LIBFDT_ENV_H
 
 #include <linux/string.h>
 
@@ -15,4 +15,4 @@ typedef __be64 fdt64_t;
 #define fdt64_to_cpu(x) be64_to_cpu(x)
 #define cpu_to_fdt64(x) cpu_to_be64(x)
 
-#endif /* _LIBFDT_ENV_H */
+#endif /* LIBFDT_ENV_H */