lib9: fix Apple disksize prototype

src/lib9/_p9dir.c

diff --git a/src/lib9/_p9dir.c b/src/lib9/_p9dir.c
index 4f9060a..afda0c5 100644
--- a/src/lib9/_p9dir.c
+++ b/src/lib9/_p9dir.c
@@ -11,7 +11,7 @@
 #define _HAVESTGEN
 #include <sys/disk.h>
 static vlong
-disksize(int fd, int dev)
+disksize(int fd, struct stat *st)
 {
 	u64int bc;
 	u32int bs;