Prepare for |is_posix| switch in the Fuchsia build in mesa/

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

Bug: 812974
Change-Id: Ibfe71ea6fdd860fc83234fa31b3223339d0b71dc
Reviewed-on: https://chromium-review.googlesource.com/988847Reviewed-by: Stephen White <senorblanco@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548219}

third_party/mesa/BUILD.gn

@@ -99,7 +99,7 @@ config("mesa_internal_config") {
     ]
   }
 
-  if (is_posix) {
+  if (is_posix || is_fuchsia) {
     defines += [
       "HAVE_DLOPEN",
       "HAVE_PTHREAD=1",