s390/oprofile: fix compile error

Fix these errors when compiling with CONFIG_OPROFILE=y and
CONFIG_PERF_EVENTS=n:
arch/s390/oprofile/init.c: In function ‘oprofile_hwsampler_start’:
arch/s390/oprofile/init.c:93:2: error: implicit declaration of function 'perf_reserve_sampling' [-Werror=implicit-function-declaration]
  retval = perf_reserve_sampling();
  ^
arch/s390/oprofile/init.c:99:3: error: implicit declaration of function 'perf_release_sampling' [-Werror=implicit-function-declaration]
   perf_release_sampling();
   ^

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/perf_event.h

@@ -87,7 +87,15 @@ struct sf_raw_sample {
 } __packed;
 
 /* Perf hardware reserve and release functions */
+#ifdef CONFIG_PERF_EVENTS
 int perf_reserve_sampling(void);
 void perf_release_sampling(void);
+#else /* CONFIG_PERF_EVENTS */
+static inline int perf_reserve_sampling(void)
+{
+	return 0;
+}
+static inline void perf_release_sampling(void) {}
+#endif /* CONFIG_PERF_EVENTS */
 
 #endif /* _ASM_S390_PERF_EVENT_H */

arch/s390/oprofile/init.c

@@ -16,6 +16,7 @@
 #include <linux/fs.h>
 #include <linux/module.h>
 #include <asm/processor.h>
+#include <asm/perf_event.h>
 
 #include "../../../drivers/oprofile/oprof.h"