Fix build with system opus

This changes GN config/target names to match what other GN
files (e.g. for media_unittests) expect.

BUG=none
R=thestig@chromium.org

Review-Url: https://codereview.chromium.org/2880333002 .
Cr-Original-Commit-Position: refs/heads/master@{#471874}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e4a5df4eea186e4d11e4cd28393db1b44458640d

linux/unbundle/opus.gn

@@ -5,7 +5,7 @@
 import("//build/config/linux/pkg_config.gni")
 import("//build/shim_headers.gni")
 
-pkg_config("system_opus") {
+pkg_config("opus_config") {
   packages = [ "opus" ]
 }
 
@@ -23,7 +23,7 @@ source_set("opus") {
   deps = [
     ":opus_shim",
   ]
-  public_configs = [ ":system_opus" ]
+  public_configs = [ ":opus_config" ]
 }
 
 source_set("opus_compare") {