powerpc/perf_counter: Add perf_counter system call on powerpc

... with an empty/dummy asm/perf_counter.h so it builds.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/include/asm/unistd.h b/arch/powerpc/include/asm/unistd.h
index e07d0c7..7cef5af 100644
--- a/arch/powerpc/include/asm/unistd.h
+++ b/arch/powerpc/include/asm/unistd.h
@@ -341,10 +341,11 @@
 #define __NR_dup3		316
 #define __NR_pipe2		317
 #define __NR_inotify_init1	318
+#define __NR_perf_counter_open	319
 
 #ifdef __KERNEL__
 
-#define __NR_syscalls		319
+#define __NR_syscalls		320
 
 #define __NR__exit __NR_exit
 #define NR_syscalls	__NR_syscalls