Add include guards to avoid multiple definitions.

Change-Id: I2a52823325a0a582a746890af7f040324d671806
diff --git a/vm/alloc/WriteBarrier.h b/vm/alloc/WriteBarrier.h
index 34b3b19..dd8f129 100644
--- a/vm/alloc/WriteBarrier.h
+++ b/vm/alloc/WriteBarrier.h
@@ -14,6 +14,9 @@
  * limitations under the License.
  */
 
+#ifndef _DALVIK_ALLOC_WRITEBARRIER
+#define _DALVIK_ALLOC_WRITEBARRIER
+
 /*
  * Note writes to the heap. These functions must be called if a field
  * of an Object in the heap changes, and before any GC safe-point. The
@@ -46,3 +49,5 @@
 {
     dvmMarkCard((Object *)obj);
 }
+
+#endif /* _DALVIK_ALLOC_WRITEBARRIER */