BUILD.gn: Make SPIRV-Cross includes relative to repo.

BUG=chromium:870747

Change-Id: I9dfdc4f88b0f17b9c2a4e102610d9eb125087bba
Reviewed-on: https://dawn-review.googlesource.com/1642
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

third_party/BUILD.gn

@@ -40,7 +40,7 @@ static_library("glad") {
 spirv_cross_dir = dawn_spirv_cross_dir
 
 config("spirv_cross_public") {
-  include_dirs = [ "." ]
+  include_dirs = [ "${spirv_cross_dir}/.." ]
   defines = [ "SPIRV_CROSS_EXCEPTIONS_TO_ASSERTIONS" ]
 }