Fix ANGLE standalone build

Referencing //third_party/SPIRV-Tools directly only works when building
inside a Chromium checkout.

Bug: angleproject:4425
Change-Id: I70ace58145a93b747a67d98b6efd4af7e3cfd370
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41628
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>

src/Pipeline/BUILD.gn

diff --git a/src/Pipeline/BUILD.gn b/src/Pipeline/BUILD.gn
index 28fe704..b85cf6a 100644
--- a/src/Pipeline/BUILD.gn
+++ b/src/Pipeline/BUILD.gn
@@ -57,11 +57,11 @@
     "..",
     "../../include",
     "../../third_party/SPIRV-Headers/include",
-    "$swiftshader_spirv_tools_dir/include",
+    "${swiftshader_spirv_tools_dir}/include",
   ]
 
   deps = [
-    "//third_party/SPIRV-Tools/src:spvtools_headers",
+    "${swiftshader_spirv_tools_dir}:spvtools_headers",
     "../../third_party/marl:Marl_headers",
     "../Device:Device_headers",
     "../System:System_headers",