riscv: xip: Fix duplicate included asm/pgtable.h

Clean up the following includecheck warning:

./arch/riscv/kernel/vmlinux-xip.lds.S: asm/pgtable.h is included more
than once.

No functional change.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>