BUILD.gn: Remove VK_USE_PLATFORM_XCB_KHR define

... for chromeos to fix `fatal error: 'xcb/xcb.h' file not found` errors.

Bug: b/209030951
Change-Id: I7b4a38a7e8363a589d202b698cde7435fed46b08
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/61548
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/Vulkan/BUILD.gn

diff --git a/src/Vulkan/BUILD.gn b/src/Vulkan/BUILD.gn
index 7f4ad97..13b19ca 100644
--- a/src/Vulkan/BUILD.gn
+++ b/src/Vulkan/BUILD.gn
@@ -18,11 +18,15 @@
 
 # Need a separate config to ensure the warnings are added to the end.
 config("swiftshader_libvulkan_private_config") {
-  if (is_linux || is_chromeos) {
+  if (is_linux) {
     defines = [
       "VK_USE_PLATFORM_XCB_KHR",
       "VK_EXPORT=__attribute__((visibility(\"default\")))",
     ]
+  } else if (is_chromeos) {
+    defines = [
+      "VK_EXPORT=__attribute__((visibility(\"default\")))",
+    ]
   } else if (is_fuchsia) {
     defines = [
       "VK_USE_PLATFORM_FUCHSIA=1",