add missing zlib dep after CL 1148633007

add missing zlib dep after CL 1148633007

This previous CL included our patched version of zlib.h, but forgot to add
the dependency in gyp/gn, making it possible to compile with the locally
patched version and link with a system version (which will fail due to
unresolved MOZ_Z_* symbols):
https://codereview.chromium.org/1148633007

BUG=466769
Committed: https://crrev.com/936c5dd07832efe4b2ef9753a9d7e99a4350d4ff
Cr-Commit-Position: refs/heads/master@{#333510}

[Mostyn Bramley-Moore, 2015-06-09 10:56:24]

mostynb@opera.com changed reviewers:
+ davidben@chromium.org, oysteine@chromium.org

[Mostyn Bramley-Moore, 2015-06-09 10:56:25]

@David, Øystein: please take a look at this zlib dependency fixup...

[davidben, 2015-06-09 15:11:14]

lgtm

[Mostyn Bramley-Moore, 2015-06-09 15:16:33]

The CQ bit was checked by mostynb@opera.com

[commit-bot: I haz the power, 2015-06-09 15:17:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174503002/20001

[commit-bot: I haz the power, 2015-06-09 16:55:36]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-09 16:56:32]

Patchset 2 (id:??) landed as
https://crrev.com/936c5dd07832efe4b2ef9753a9d7e99a4350d4ff
Cr-Commit-Position: refs/heads/master@{#333510}