vcl: explicitly qualify ICU types with icu:: namespace

It will be required by ICU 61 anyway, see
https://ssl.icu-project.org/repos/icu/trunk/icu4c/readme.html#RecBuild

Change-Id: Iecb30b903e9a67252147a8cc78c641621d763755

vcl/inc/scrptrun.h

@@ -58,7 +58,7 @@ struct ParenStackEntry
     }
 };
 
-class ScriptRun : public UObject {
+class ScriptRun : public icu::UObject {
 public:
 
     ScriptRun(const UChar chars[], int32_t length);