Add :build_flags dep to //gpu/vulkan:vulkan

This fixes an dependency problem where build_flags headers may be missing
causing build flakiness.

Bug: 1106917
Change-Id: If15a0e54055e7fd9d84cbba119b4f03e262b71d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2304102Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: Maciek Kumorek <makumo@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#790944}

gpu/vulkan/BUILD.gn

@@ -107,6 +107,7 @@ if (enable_vulkan) {
     defines = [ "IS_VULKAN_IMPL" ]
 
     deps = [
+      ":buildflags",
       "//base",
       "//base/util/type_safety",
       "//components/crash/core/common:crash_key",