Add optional tags to expat.

Change-Id: I356154b04a356a51a532a691d918c497c421586b
diff --git a/Android.mk b/Android.mk
index 08cd0e7..bcba01c 100644
--- a/Android.mk
+++ b/Android.mk
@@ -32,7 +32,7 @@
 endif
 
 LOCAL_MODULE:= libexpat
-
+LOCAL_MODULE_TAGS := optional
 LOCAL_COPY_HEADERS_TO := $(common_COPY_HEADERS_TO)
 LOCAL_COPY_HEADERS := $(common_COPY_HEADERS)
 
@@ -49,7 +49,7 @@
 LOCAL_C_INCLUDES += $(common_C_INCLUDES)
 
 LOCAL_MODULE:= libexpat
-
+LOCAL_MODULE_TAGS := optional
 LOCAL_COPY_HEADERS_TO := $(common_COPY_HEADERS_TO)
 LOCAL_COPY_HEADERS := $(common_COPY_HEADERS)