Remove wrong include path for vulkan_tests

/usr/include should never be needed explicitly, and conflicts with the
sysroot

Bug: None
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ie07580c98136e9b0023ba7b650a0364397ff38fb
Reviewed-on: https://chromium-review.googlesource.com/1040737Reviewed-by: Chris Blume <cblume@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555567}

gpu/vulkan/BUILD.gn

@@ -81,7 +81,6 @@ if (enable_vulkan) {
       "tests/vulkan_tests_main.cc",
     ]
 
-    include_dirs = [ "/usr/include" ]
     if (current_cpu == "x64") {
       lib_dirs = [
         "/usr/lib/x86_64-linux-gnu",