treewide: remove double forward declaration of read_in_full

cache.h's nature of a dumping ground of includes prevented it from
being included in some compat/ files, forcing us into a workaround
of having a double forward declaration of the read_in_full() function
(see commit 14086b0a13 ("compat/pread.c: Add a forward declaration to
fix a warning", 2007-11-17)).  Now that we have moved functions like
read_in_full() from cache.h to wrapper.h, and wrapper.h isn't littered
with unrelated and scary #defines, get rid of the extra forward
declaration and just have compat/pread.c include wrapper.h.

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-write.c

@@ -10,6 +10,7 @@
 #include "oidmap.h"
 #include "pack-objects.h"
 #include "pack-revindex.h"
+#include "wrapper.h"
 
 void reset_pack_idx_option(struct pack_idx_option *opts)
 {