gn: Don't let libyuv depend on libjpeg_turbo on iOS.

This dependency doesn't exist in the gyp build either, and it causes
problems on builds building 'all'.

BUG=633316
R=kjellander@chromium.org

Review URL: https://codereview.chromium.org/2201963002 .

BUILD.gn

@@ -82,15 +82,13 @@ static_library("libyuv") {
    public_configs = [ ":libyuv_config" ]
 
   defines = []
+  deps = []
 
   if (!is_ios) {
     defines += [ "HAVE_JPEG" ]
+    deps += [ "//third_party:jpeg" ]
   }
 
-  deps = [
-    "//third_party:jpeg",
-  ]
-
   if (use_neon) {
     deps += [ ":libyuv_neon" ]
   }