Fix Vulkan Swiftshader build on macOS

There was a typo in macro name used to do platform detection.

Bug:

Change-Id: I81958419b4283d1d0498f5edc48efc385a648e09
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15440
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/dawn_native/vulkan/BackendVk.cpp b/src/dawn_native/vulkan/BackendVk.cpp
index 75402dd..d2aaa76 100644
--- a/src/dawn_native/vulkan/BackendVk.cpp
+++ b/src/dawn_native/vulkan/BackendVk.cpp
@@ -27,7 +27,7 @@
 constexpr char kSwiftshaderLibName[] = "libvk_swiftshader.so";
 #    elif defined(DAWN_PLATFORM_WINDOWS)
 constexpr char kSwiftshaderLibName[] = "vk_swiftshader.dll";
-#    elif defined(DAWN_PLATFORM_MAC)
+#    elif defined(DAWN_PLATFORM_MACOS)
 constexpr char kSwiftshaderLibName[] = "libvk_swiftshader.dylib";
 #    else
 #        error "Unimplemented Swiftshader Vulkan backend platform"