Don't define VK_USE_PLATFORM_XCB_KHR for Linux-on-Fuchsia

Change-Id: Ia2d9238ab24f1474ed1ff53f44bbb36fed1a97fc
diff --git a/BUILD.gn b/BUILD.gn
index 5c70229..8066e36 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -20,7 +20,8 @@
     defines = [ "VK_USE_PLATFORM_WIN32_KHR" ]
   }
   if (is_linux) {
-    defines = [ "VK_USE_PLATFORM_XCB_KHR" ]
+    # Linux-on-Fuchsia doesn't want XCB
+    #defines = [ "VK_USE_PLATFORM_XCB_KHR" ]
   }
   if (is_android) {
     defines = [ "VK_USE_PLATFORM_ANDROID_KHR" ]