riscv: fix -Wundef warning for CONFIG_RISCV_BOOT_SPINWAIT

riscv: fix -Wundef warning for CONFIG_RISCV_BOOT_SPINWAIT

Since commit 80b6093b55e3 ("kbuild: add -Wundef to KBUILD_CPPFLAGS
for W=1 builds"), building with W=1 detects misuse of #if.

  $ make W=1 ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- arch/riscv/kernel/
    [snip]
    AS      arch/riscv/kernel/head.o
  arch/riscv/kernel/head.S:329:5: warning: "CONFIG_RISCV_BOOT_SPINWAIT" is not defined, evaluates to 0 [-Wundef]
    329 | #if CONFIG_RISCV_BOOT_SPINWAIT
        |     ^~~~~~~~~~~~~~~~~~~~~~~~~~

CONFIG_RISCV_BOOT_SPINWAIT is a bool option. #ifdef should be used.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Fixes: 2ffc48fc7071 ("RISC-V: Move spinwait booting method to its own config")
Link: https://lore.kernel.org/r/20230106161213.2374093-1-masahiroy@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>