bdiff: include compat.h in header to define ssize_t

Before ff4c9c6263de, compat.h was included first so it happened to work.
But we shouldn't rely on the include order.

mercurial/bdiff.h

@@ -1,6 +1,8 b''
 #ifndef _HG_BDIFF_H_
 #define _HG_BDIFF_H_
 
+#include "compat.h"
+
 struct bdiff_line {
 	int hash, n, e;
 	ssize_t len;