Fix pkt-line.h to compile with a non-GCC compiler

pkt-line.h uses GCC's __attribute__ extension but does not include
git-compat-util.h.  So it will not compile with a compiler that does
not support this extension.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

pkt-line.h

 #ifndef PKTLINE_H
 #define PKTLINE_H
 
+#include "git-compat-util.h"
+
 /*
  * Silly packetized line writing interface
  */