[X11] Don't redefine VK_USE_PLATFORM_XLIB_KHR

This is required to prevent conflicts when defining the variable from
gn. The "ifndef" guard matches the others found in this file.

Bug: chromium:345261080

Change-Id: I73a4475f62d13cbe0595a91f619382a95a58e2a5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/229494
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Loko Kung <lokokung@google.com>
Auto-Submit: Tom Anderson <thomasanderson@google.com>
Reviewed-by: Brandon Jones <bajones@chromium.org>

src/dawn/common/vulkan_platform.h

diff --git a/src/dawn/common/vulkan_platform.h b/src/dawn/common/vulkan_platform.h
index 18b9ad2..99a2f9d 100644
--- a/src/dawn/common/vulkan_platform.h
+++ b/src/dawn/common/vulkan_platform.h
@@ -149,7 +149,9 @@
 #endif  // DAWN_PLATFORM_IS(WINDOWS)
 
 #if defined(DAWN_USE_X11)
+#ifndef VK_USE_PLATFORM_XLIB_KHR
 #define VK_USE_PLATFORM_XLIB_KHR
+#endif
 #ifndef VK_USE_PLATFORM_XCB_KHR
 #define VK_USE_PLATFORM_XCB_KHR
 #endif