Add SPIR-Tools include headers path for src/Pipeline

Will be used by SpirvShaderEnumNames.cpp in https://swiftshader-review.googlesource.com/c/SwiftShader/+/41510

Bug: angleproject:4425
Change-Id: I631c593432c12a6ccf64edcb83b2e11ac87104c0
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41588
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
diff --git a/src/Pipeline/BUILD.gn b/src/Pipeline/BUILD.gn
index 00747b5..e1d99fd 100644
--- a/src/Pipeline/BUILD.gn
+++ b/src/Pipeline/BUILD.gn
@@ -57,6 +57,8 @@
     "..",
     "../../include",
     "../../third_party/SPIRV-Headers/include",
+    "$swiftshader_spirv_tools_dir/include",
+    "$spirv_tools_spirv_headers_dir/include",
   ]
 
   deps = [