Prepare for |is_posix| switch in the Fuchsia build

|is_posix| will be switched to false for Fuchsia, this is a preliminary change.

Bug: chromium:812974
Change-Id: I3bfda3e056ad1e5229834286ce5d095d9204a428
Reviewed-on: https://webrtc-review.googlesource.com/65782
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#22753}

modules/desktop_capture/BUILD.gn

@@ -391,7 +391,7 @@ if (use_desktop_capture_differ_sse2) {
       "differ_vector_sse2.h",
     ]
 
-    if (is_posix) {
+    if (is_posix || is_fuchsia) {
       cflags = [ "-msse2" ]
     }
   }