[POWERPC] Fix compile breakage for 64-bit UP configs

The rearrangements in 945feb17
("[POWERPC] irqtrace support for 64-bit powerpc") caused 64-bit
non-SMP configs to fail to compile with a message about
local_irq_save being undefined in include/linux/proportions.h.
This follows the lead of x86 in including <linux/irqflags.h> in
asm/system.h, which fixes the problem.
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/system.h

@@ -5,6 +5,7 @@
 #define _ASM_POWERPC_SYSTEM_H
 
 #include <linux/kernel.h>
+#include <linux/irqflags.h>
 
 #include <asm/hw_irq.h>