[build] Fix missing import

Fix after https://crrev.com/c/2315691, which references
mac_xcode_version without importing gclient_args.gni where it gets
defined. It only worked in the Chromium project since the import
is in Chromium's build overrides.

Bug: 1109624
Change-Id: I575c7b87dbb96c3a79b8243bfe63c33b2427479d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317871Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791796}

build/config/compiler/BUILD.gn

@@ -11,6 +11,7 @@ import("//build/config/clang/clang.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/coverage/coverage.gni")
 import("//build/config/dcheck_always_on.gni")
+import("//build/config/gclient_args.gni")
 import("//build/config/host_byteorder.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")