[fuchsia]: Fix Fuchsia build.

A previous CL broke the build because a GLFW-related config
is no longer exposed. This fixes the issue.

Bug=dawn:221

Change-Id: I83d43b55d58d18e04ea20f732c76830a798f2487
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/11181
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: David Turner <digit@google.com>

BUILD.gn

@@ -593,7 +593,8 @@ if (is_win || is_linux || is_mac) {
   # The mock implementation of GLFW on Fuchsia
   config("dawn_glfw_public_config") {
     # Allow inclusion of <GLFW/glfw3.h>
-    configs = [ "third_party:glfw_public" ]
+    include_dirs = [ "${dawn_glfw_dir}/include" ]
+
 
     # The GLFW/glfw3.h header includes <GL/gl.h> by default, but the latter
     # does not exist on Fuchsia. Defining GLFW_INCLUDE_NONE helps work around