Fix V4L2VideoEncodeAccelerator is not enabled in GN build

media/gpu/BUILD.gn

Index: media/gpu/BUILD.gn
diff --git a/media/gpu/BUILD.gn b/media/gpu/BUILD.gn
index 31e23274e63697e40fa8028caad5412188f4e792..b928d0b960e87724a33724b5768156f394745526 100644
--- a/media/gpu/BUILD.gn
+++ b/media/gpu/BUILD.gn
@@ -131,6 +131,9 @@ config("gpu_config") {
   if (is_chromeos && use_v4lplugin) {
     defines += [ "USE_LIBV4L2" ]
   }
+  if (is_chromeos && use_v4l2_codec) {
+    defines += [ "USE_V4L2_CODEC" ]
+  }
 }
 
 component("gpu") {
@@ -257,7 +260,6 @@ component("gpu") {
       deps += [ ":libv4l2_generate_stubs" ]
     }
     if (use_v4l2_codec) {
-      defines += [ "USE_V4L2_CODEC" ]
       deps += [ "//third_party/libyuv" ]
       sources += [
         "generic_v4l2_device.cc",