perf/x86: Fix missing struct before structure name

When CONFIG_PERF_EVENTS disabled, there will have a compiliation
error, because missing struct before structure name.

Signed-off-by: Jovi Zhang <bookjovi@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jiri Kosina <trivial@kernel.org>
Link: http://lkml.kernel.org/r/CACV3sbKF%3DCX%2B2jWEWesfCA6rBoQ3wDM4-5ac9MuBtVbCtMRHdQ@mail.gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/perf_event.h

@@ -234,7 +234,7 @@ extern struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr);
 extern void perf_get_x86_pmu_capability(struct x86_pmu_capability *cap);
 extern void perf_check_microcode(void);
 #else
-static inline perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
+static inline struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
 {
 	*nr = 0;
 	return NULL;