[build] Include vulkan_headers as a dependency target.

This ensures that the Vulkan headers and header macros are defined
properly.

Test: fx build
Change-Id: I1ceb0704f30d0e8ff67656e3fd4087f369670288
diff --git a/BUILD.gn b/BUILD.gn
index 8ee689c..b6be3c2 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -181,7 +181,6 @@
     include_dirs += [
       "fuchsia/include",
       "platform/include",
-      "//third_party/Vulkan-Headers/include",
     ]
 
     deps = [
@@ -194,6 +193,7 @@
       "//sdk/lib/zxio",
       "//src/devices/bind/fuchsia.goldfish.platform.sysmem.heap:fuchsia.goldfish.platform.sysmem.heap_cpp",
       "//src/zircon/lib/zircon",
+      "//third_party/Vulkan-Headers:vulkan_headers",
       "//zircon/system/ulib/sysmem-version",
       "//zircon/system/ulib/trace:trace-with-static-engine",
       "//zircon/system/ulib/trace-provider:trace-provider-with-static-engine",