Fix disable_nacl=1 build.

This was broken in https://codereview.chromium.org/61763014: gyp complains that
native_client_sdk/native_client_sdk_untrusted.gyp, included from build/all.gyp,
has no targets.

BUG=319174
R=binji@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/66163013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235114 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235114