ART: Clean up includes.

Reduce dependencies to improve incremental build times.
Break up circular dependency involving class_linker-inl.h.

Change-Id: I4be742c5c2b5cd9855beea86630fd68aab76b0db
diff --git a/runtime/gc/allocator/rosalloc.h b/runtime/gc/allocator/rosalloc.h
index d1e7ad9..a54edcc 100644
--- a/runtime/gc/allocator/rosalloc.h
+++ b/runtime/gc/allocator/rosalloc.h
@@ -30,11 +30,13 @@
 #include "base/mutex.h"
 #include "base/logging.h"
 #include "globals.h"
-#include "mem_map.h"
 #include "thread.h"
 #include "utils.h"
 
 namespace art {
+
+class MemMap;
+
 namespace gc {
 namespace allocator {