EmbeddedPkg/libfdt: Add strcmp and strncpy to libfdt_env.h

EmbeddedPkg/libfdt: Add strcmp and strncpy to libfdt_env.h

OpenSBI has started using those in v0.9. See:
https://github.com/riscv/opensbi/blob/v0.9/lib/utils/fdt/fdt_domain.c

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Cc: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Acked-by: Leif Lindholm <leif@nuviainc.com>