[IA64] fix UP build

Put back a secondary declaration of ipi_base_addr in hardirq.h.  It
really is needed for UP builds and there doesn't seem to be an easy
way to avoid include-hell otherwise.

Signed-off-by: davidm@hpl.hp.com
Signed-off-by: Tony Luck <tony.luck@intel.com>

include/asm-ia64/hardirq.h

@@ -60,4 +60,6 @@
 # error HARDIRQ_BITS is too low!
 #endif
 
+extern void __iomem *ipi_base_addr;
+
 #endif /* _ASM_IA64_HARDIRQ_H */