page-read: Fix compilation on older distros

SEEK_DATA appeared quite recently, define it if not present
in system headers.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

page-read.c

@@ -9,6 +9,11 @@
 #include "protobuf.h"
 #include "protobuf/pagemap.pb-c.h"
 
+#ifndef SEEK_DATA
+#define SEEK_DATA	3
+#define SEEK_HOLE	4
+#endif
+
 static int get_page_vaddr(struct page_read *pr, struct iovec *iov)
 {
 	int ret;