Fix build for iOS after "Make nanobench and dm be usable from Chromium build"

Fix build for iOS after "Make nanobench and dm be usable from Chromium
build"

BUG=skia:2992

Review URL: https://codereview.chromium.org/716413003
diff --git a/gyp/iOSShell.gyp b/gyp/iOSShell.gyp
index ac2ed30..2a9698a 100644
--- a/gyp/iOSShell.gyp
+++ b/gyp/iOSShell.gyp
@@ -67,6 +67,7 @@
             '../experimental/iOSSampleApp/Shared',
             '../include/utils/ios',
             '../src/views/mac',
+            '../bench',
           ],
           'xcode_settings' : {
             'INFOPLIST_FILE' : '../experimental/iOSShell/iOSShell-Info.plist',