libyuv: Include foo_posix.cc in the gn build, like it's done in the gyp build.

This is required to get things linking in clang/win. Medium-term, we should
clean libyuv up to not require _posix files on non-posix.

Reviewed at https://webrtc-codereview.appspot.com/52069004/

BUG=496928

Review URL: https://webrtc-codereview.appspot.com/54689004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1423 16f28f9a-4ce2-e073-06de-1de4eb20be90

BUILD.gn

@@ -17,7 +17,12 @@ config("libyuv_config") {
 
 use_neon = current_cpu == "arm" && (arm_use_neon || arm_optionally_use_neon)
 
-source_set("libyuv") {
+static_library("libyuv") {
+  # libyuv requires that foo_posix.cc is linked in on Windows too,
+  # just like in libyuv.gyp
+  # TODO(thakis): Fix this, http://crbug.com/496928
+  set_sources_assignment_filter([])
+
   sources = [
     "include/libyuv.h",
     "include/libyuv/basic_types.h",