perf_counter: include missing header

Impact: build fix

In order to compile a kernel with performance counter patches,
<asm/irq_regs.h> has to be included to provide the declaration of
struct pt_regs *get_irq_regs(void);

[ This bug was masked by unrelated x86 header file changes in the
  x86 tree, but occurs in the tip:perfcounters/core standalone
  tree. ]

Signed-off-by: Tim Blechmann <tim@klingt.org>
Orig-LKML-Reference: <20090314142925.49c29c17@thinkpad>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_counter.c

@@ -24,6 +24,8 @@
 #include <linux/vmstat.h>
 #include <linux/rculist.h>
 
+#include <asm/irq_regs.h>
+
 /*
  * Each CPU has a list of per CPU counters:
  */