Update GYP file to depend on zlib.

(Trying to get Windows to build via try servers so there could be a few of
these changes before I'm done.)

git-svn-id: http://ots.googlecode.com/svn/trunk@30 a4e77c2c-9104-11de-800e-5b313e0d2bf3
diff --git a/ots.gyp b/ots.gyp
index 300fbf6..a7c073e 100644
--- a/ots.gyp
+++ b/ots.gyp
@@ -64,6 +64,9 @@
           'include',
         ],
       },
+      'dependencies': [
+        '../zlib/zlib.gyp:zlib',
+      ],
     },
   ],
 }