Desktop capture: Fix GN target and exclude differ* sources on iOS

webrtc/modules/desktop_capture/BUILD.gn

Index: webrtc/modules/desktop_capture/BUILD.gn
diff --git a/webrtc/modules/desktop_capture/BUILD.gn b/webrtc/modules/desktop_capture/BUILD.gn
index 735b4605d6ab6c17453397426163e437a7ff4d41..61a3de2fe16379bccc07d2dd054fb62087e39dd2 100644
--- a/webrtc/modules/desktop_capture/BUILD.gn
+++ b/webrtc/modules/desktop_capture/BUILD.gn
@@ -38,13 +38,8 @@ source_set("desktop_capture") {
     "desktop_capture_options.cc",
     "desktop_capture_options.h",
     "desktop_capturer.h",
-    "desktop_capturer.h",
     "desktop_frame_win.cc",
     "desktop_frame_win.h",
-    "differ.cc",
-    "differ.h",
-    "differ_block.cc",
-    "differ_block.h",
     "mac/desktop_configuration.h",
     "mac/desktop_configuration.mm",
     "mac/desktop_configuration_monitor.cc",
@@ -129,6 +124,15 @@ source_set("desktop_capture") {
     ]
   }
 
+  if (!is_ios) {
+    sources += [
+      "differ.cc",
+      "differ.h",
+      "differ_block.cc",
+      "differ_block.h",
+    ]
+  }
+
   if (is_mac) {
     libs = [
       "AppKit.framework",