Label /proc/config.gz

Add a label to /proc/config.gz, so we can distinguish this file from
other /proc files in security policy.

For now, only init is allowed read access. All others are denied.
TODO: clarify exactly who needs access. Further access will be granted
in a future commit.

Bug: 35126415
Test: policy compiles and no device boot problems.
Change-Id: I8b480890495ce5b8aa3f8c7eb00e14159f177860
diff --git a/private/file.te b/private/file.te
index 818a53d..da5f9ad 100644
--- a/private/file.te
+++ b/private/file.te
@@ -2,3 +2,6 @@
 typealias audio_data_file alias audio_firmware_file;
 typealias app_data_file alias platform_app_data_file;
 typealias app_data_file alias download_file;
+
+# /proc/config.gz
+type config_gz, fs_type;