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

@@ -1,21 +1,23 b''
 #ifndef _HG_BDIFF_H_
 #define _HG_BDIFF_H_
 
+#include "compat.h"
+
 struct bdiff_line {
 	int hash, n, e;
 	ssize_t len;
 	const char *l;
 };
 
 struct bdiff_hunk;
 struct bdiff_hunk {
 	int a1, a2, b1, b2;
 	struct bdiff_hunk *next;
 };
 
 int bdiff_splitlines(const char *a, ssize_t len, struct bdiff_line **lr);
 int bdiff_diff(struct bdiff_line *a, int an, struct bdiff_line *b, int bn,
                struct bdiff_hunk *base);
 void bdiff_freehunks(struct bdiff_hunk *l);
 
 #endif