Fix build with use_system_zlib=1

http://codereview.chromium.org/469018

git-svn-id: http://src.chromium.org/svn/trunk/src/build@34067 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

linux/system.gyp

@@ -69,6 +69,7 @@
             ['use_system_ssl==0', {
               'dependencies': [
                 '../../net/third_party/nss/nss.gyp:ssl',
+                '../../third_party/zlib/zlib.gyp:zlib',
               ],
               'direct_dependent_settings': {
                 'cflags': [