s390/irq: Only define synchronize_irq() on SMP

In uniprocessor configurations, synchronize_irq() is defined in
<linux/hardirq.h> as a macro, and this function definition fails to
compile.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: stable@vger.kernel.org # 3.9
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>