Add a tag for tracing system server.

This change is needed for
https://android-review.googlesource.com/#/c/157562/

BUG: 21739901
Change-Id: Ia6b9414de82c210f3d7a275d21b8cd00bd19dcdf
diff --git a/include/cutils/trace.h b/include/cutils/trace.h
index 6d9b3bc..27296f3 100644
--- a/include/cutils/trace.h
+++ b/include/cutils/trace.h
@@ -68,7 +68,8 @@
 #define ATRACE_TAG_BIONIC           (1<<16)
 #define ATRACE_TAG_POWER            (1<<17)
 #define ATRACE_TAG_PACKAGE_MANAGER  (1<<18)
-#define ATRACE_TAG_LAST             ATRACE_TAG_PACKAGE_MANAGER
+#define ATRACE_TAG_SYSTEM_SERVER    (1<<19)
+#define ATRACE_TAG_LAST             ATRACE_TAG_SYSTEM_SERVER
 
 // Reserved for initialization.
 #define ATRACE_TAG_NOT_READY        (1LL<<63)