pack-objects: mark file-local variable static

old_try_to_free_routine is not meant for use from other files.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -1529,7 +1529,7 @@ static void try_to_free_from_threads(size_t size)
 	read_unlock();
 }
 
-try_to_free_t old_try_to_free_routine;
+static try_to_free_t old_try_to_free_routine;
 
 /*
  * The main thread waits on the condition that (at least) one of the workers