🐳 Move linux pkg_config() calls into separate BUILD.gn files

build/config/linux/libffi/BUILD.gn

Index: build/config/linux/libffi/BUILD.gn
diff --git a/third_party/vulkan/BUILD.gn b/build/config/linux/libffi/BUILD.gn
similarity index 63%
copy from third_party/vulkan/BUILD.gn
copy to build/config/linux/libffi/BUILD.gn
index b7388a2ddeb51f6b9e87a143fe1ac642be4cc4e5..a4041727b1c0908732c35045c0a9fff15304b868 100644
--- a/third_party/vulkan/BUILD.gn
+++ b/build/config/linux/libffi/BUILD.gn
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-config("vulkan_headers") {
-  include_dirs = [ "include" ]
+import("//build/config/linux/pkg_config.gni")
+
+pkg_config("libffi") {
+  packages = [ "libffi" ]
 }