Add /dev/kmsg_debug.

Add /dev/kmsg_debug on userdebug devices, to allow crash_dump to log
crashes to dmesg when logd isn't up yet (or is the one crashing).

(Originally commited in a015186fab804b4cc8f7d26221451b5340db11ce)
Bug: 36574794
Bug: 62101480
Test: Builds and boots.
Change-Id: I249e11291c58fee77098dec3fd3271ea23363ac9
diff --git a/private/file_contexts b/private/file_contexts
index 40b66cd..85080b5 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -107,6 +107,7 @@
 /dev/ptmx		u:object_r:ptmx_device:s0
 /dev/pvrsrvkm		u:object_r:gpu_device:s0
 /dev/kmsg		u:object_r:kmsg_device:s0
+/dev/kmsg_debug	u:object_r:kmsg_debug_device:s0
 /dev/null		u:object_r:null_device:s0
 /dev/nvhdcp1		u:object_r:video_device:s0
 /dev/random		u:object_r:random_device:s0