Fix build with system libpng

This updates the unbundle file to get back in sync with mainline,
so that targets that depend on libpng_config continue to work.

This is similar to recently landed https://codereview.chromium.org/2808383006 .

This does not affect any build at Google. build/linux/unbundle
is a completely parallel directory tree intended to be used
by Linux distribution packagers. Please see
https://cs.chromium.org/chromium/src/build/linux/unbundle/README
for more context.

BUG=none
R=thestig@chromium.org

Review-Url: https://codereview.chromium.org/2888783003 .
Cr-Original-Commit-Position: refs/heads/master@{#472530}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fe19263cfa7e714a402a3ed9ed59d304a3da8162

linux/unbundle/libpng.gn

@@ -5,7 +5,7 @@
 import("//build/config/linux/pkg_config.gni")
 import("//build/shim_headers.gni")
 
-pkg_config("system_libpng") {
+pkg_config("libpng_config") {
   packages = [ "libpng" ]
 }
 
@@ -21,5 +21,5 @@ source_set("libpng") {
   deps = [
     ":libpng_shim",
   ]
-  public_configs = [ ":system_libpng" ]
+  public_configs = [ ":libpng_config" ]
 }