MIPS: paravirt: Fix undefined reference to smp_bootstrap

MIPS: paravirt: Fix undefined reference to smp_bootstrap

If the paravirt machine is compiles without CONFIG_SMP, the following
linker error occurs

arch/mips/kernel/head.o: In function `kernel_entry':
(.ref.text+0x10): undefined reference to `smp_bootstrap'

due to the kernel entry macro always including SMP startup code.
Wrap this code in CONFIG_SMP to fix the error.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org # 3.16+
Patchwork: https://patchwork.linux-mips.org/patch/14212/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>