lib: utils: Add strncpy macro to libfdt_env.h

we want to use sbi_strncpy as strncpy in the OpenSBI implementation for
libfdt.

Just like 2845d2d2



Signed-off-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>