Make the appropriate cast to protect from mingw.

As the comment at the top of the file says:

    STATUST: mingw does seem to redefine UNKNOWN_ERROR from our enum
    value, so a cast is necessary.

Bug: http://b/26523949
Change-Id: I9d86cafdc47bbb9ce0e967cdde9315babc083432
diff --git a/tools/aapt/Resource.cpp b/tools/aapt/Resource.cpp
index a4e5d3d..1b30d36 100644
--- a/tools/aapt/Resource.cpp
+++ b/tools/aapt/Resource.cpp
@@ -2107,7 +2107,7 @@
                 indentStr);
     }
 
-    return hasErrors ? UNKNOWN_ERROR : NO_ERROR;
+    return hasErrors ? STATUST(UNKNOWN_ERROR) : NO_ERROR;
 }
 
 static status_t writeResourceLoadedCallback(