avoid dup symbol for os x

src/libdiskfs/file.c

@@ -2,6 +2,8 @@
 #include <libc.h>
 #include <diskfs.h>
 
+int nfilereads;
+
 typedef struct DiskFile DiskFile;
 struct DiskFile
 {
@@ -9,8 +11,6 @@ struct DiskFile
 	int fd;
 };
 
-int nfilereads;
-
 static long
 preadn(int fd, void *vdata, u32int ulen, u64int offset)
 {

src/libdiskfs/venti.c

@@ -13,7 +13,7 @@ struct DiskVenti
 	VtCache *c;
 };
 
-int nfilereads;
+extern int nfilereads;
 
 /*
  * This part is like file.c but doesn't require storing the root block