pack: move use_pack()

The function open_packed_git() needs to be temporarily made global. Its
scope will be restored to static in a subsequent commit.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

streaming.c

@@ -3,6 +3,7 @@
  */
 #include "cache.h"
 #include "streaming.h"
+#include "packfile.h"
 
 enum input_source {
 	stream_error = -1,