Add SPIRV-Tools headers dependency

And remove external SPIRV-Headers dependency. We use an internal copy.

Bug: angleproject:4425
Change-Id: I36c8c836197d6642d4c696520d61bdf7790fce6c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41589
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>

src/Pipeline/BUILD.gn

diff --git a/src/Pipeline/BUILD.gn b/src/Pipeline/BUILD.gn
index e1d99fd..28fe704 100644
--- a/src/Pipeline/BUILD.gn
+++ b/src/Pipeline/BUILD.gn
@@ -58,10 +58,10 @@
     "../../include",
     "../../third_party/SPIRV-Headers/include",
     "$swiftshader_spirv_tools_dir/include",
-    "$spirv_tools_spirv_headers_dir/include",
   ]
 
   deps = [
+    "//third_party/SPIRV-Tools/src:spvtools_headers",
     "../../third_party/marl:Marl_headers",
     "../Device:Device_headers",
     "../System:System_headers",