ART: Forward-declare SuspendReason in thread.h

Reduce transitive includes, iwyu.

Test: mmma art
Change-Id: Ief9708af95db988f2b77accdd144b3d90e4248d2
diff --git a/runtime/thread.h b/runtime/thread.h
index 7c8ad24..4991566 100644
--- a/runtime/thread.h
+++ b/runtime/thread.h
@@ -41,7 +41,6 @@
 #include "read_barrier_config.h"
 #include "runtime_globals.h"
 #include "runtime_stats.h"
-#include "suspend_reason.h"
 #include "thread_state.h"
 
 class BacktraceMap;
@@ -96,6 +95,7 @@
 class ShadowFrame;
 class SingleStepControl;
 class StackedShadowFrameRecord;
+enum class SuspendReason : char;
 class Thread;
 class ThreadList;
 enum VisitRootFlags : uint8_t;