sparse: Fix errors and silence warnings

* load_file() returns a void pointer but is using 0 for the return
   value

 * builtin/receive-pack.c forgot to include builtin.h

 * packet_trace_prefix can be marked static

 * ll_merge takes a pointer for its last argument, not an int

 * crc32 expects a pointer as the second argument but Z_NULL is defined
   to be 0 (see 38f4d13 sparse fix: Using plain integer as NULL pointer,
   2006-11-18 for more info)

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -1,4 +1,4 @@
-#include "cache.h"
+#include "builtin.h"
 #include "pack.h"
 #include "refs.h"
 #include "pkt-line.h"