Fixing externalLinkTarget. isExternalLinkTargetSet was set to

wrong sharedUtil method.

src/display/annotation_layer.js

@@ -31,7 +31,7 @@
 var AnnotationBorderStyleType = sharedUtil.AnnotationBorderStyleType;
 var AnnotationType = sharedUtil.AnnotationType;
 var Util = sharedUtil.Util;
-var isExternalLinkTargetSet = sharedUtil.Util;
+var isExternalLinkTargetSet = sharedUtil.isExternalLinkTargetSet;
 var LinkTargetStringMap = sharedUtil.LinkTargetStringMap;
 var warn = sharedUtil.warn;
 var CustomStyle = displayDOMUtils.CustomStyle;