Runtime::PreZygoteFork returns void, not boolean.

This method aborts on failure (as it should) and unconditionally
returns true, so making it return void simplifies callers.

Change-Id: Iae39bd327f20311579ece47efa8afd1be7defce9
diff --git a/runtime/runtime.h b/runtime/runtime.h
index ed60d4d..8c3ef0e 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -355,7 +355,7 @@
 
   void SetStatsEnabled(bool new_state);
 
-  bool PreZygoteFork();
+  void PreZygoteFork();
   bool InitZygote();
   void DidForkFromZygote();