Fix Zlib Win GN build after https://codereview.chromium.org/742583003

TBR=thakis

Review URL: https://codereview.chromium.org/738103002
diff --git a/third_party/zlib/BUILD.gn b/third_party/zlib/BUILD.gn
index 810fbb7..b4d9cfd 100644
--- a/third_party/zlib/BUILD.gn
+++ b/third_party/zlib/BUILD.gn
@@ -57,7 +57,7 @@
     "zutil.h",
   ]
 
-  if (!is_win && (cpu_arch == "x86" || cpu_arch == "x64")) {
+  if (!is_ios && (cpu_arch == "x86" || cpu_arch == "x64")) {
     sources += [ "x86.c" ]
   }