fix enabled-mergelibs-more build

ErrCode::toString is needed by desktop/source/app/ unit tests now

Change-Id: Ie7a9e67aca763c958d08f3826bca5df4e0d828f3
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/175769
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Jenkins

include/comphelper/errcode.hxx

@@ -137,8 +137,8 @@ public:
     }
 
     /// Return a string suitable for debug output, the same as the operator<< function
-    UNLESS_MERGELIBS(COMPHELPER_DLLPUBLIC) OUString toString() const;
-    
+    COMPHELPER_DLLPUBLIC OUString toString() const;
+
     template <typename... Args> bool anyOf(Args... args) const
     {
         static_assert(sizeof...(args) > 0);