git-compat-util: move include of "compat/zlib.h" into "git-zlib.h"

We include "compat/zlib.h" in "git-compat-util.h", which is
unnecessarily broad given that we only have a small handful of files
that use the zlib library. Move the header into "git-zlib.h" instead and
adapt users of zlib to include that header.

One exception is the reftable library, as we don't want to use the
Git-specific wrapper of zlib there, so we include "compat/zlib.h"
instead. Furthermore, we move the include into "reftable/system.h" so
that users of the library other than Git can wire up zlib themselves.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/block.c

@@ -13,7 +13,6 @@ https://developers.google.com/open-source/licenses/bsd
 #include "record.h"
 #include "reftable-error.h"
 #include "system.h"
-#include "compat/zlib-compat.h"
 
 int header_size(int version)
 {