StyleSheetUser use in dynamic_cast should better be SAL_DLLPUBLIC_RTTI

...so that the dynamic_cast is guaranteed to work on macOS (where RTTI
equivalence is determined by address, not by strcmp).  The relevant dynamic_cast
is in SdStyleSheet::IsUsed in sd/source/core/stlsheet.cxx, and at least during a
`make check` it appears to only be fed SdStyleSheet instances derived from
StyleSheetUser (and SdStyleSheet is also defiined in Library_sd, so the
dynamic_cast happens to work fine also on macOS in that case).  But better be
conservative and mark StyleSheetUser as SAL_DLLPUBLIC_RTTI after all.

Change-Id: I760f58357b7fe3b4826ea496934e03da3499419e
Reviewed-on: https://gerrit.libreoffice.org/50246Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>

include/svl/stylesheetuser.hxx

@@ -10,6 +10,10 @@
 #ifndef INCLUDED_SVL_STYLESHEETUSER_HXX
 #define INCLUDED_SVL_STYLESHEETUSER_HXX
 
+#include <sal/config.h>
+
+#include <sal/types.h>
+
 namespace svl
 {
 
@@ -19,7 +23,7 @@ namespace svl
   a SfxStyleSheet (and listen on it). It can be queried by the stylesheet
   to determine if it is really used.
   */
-class StyleSheetUser
+class SAL_DLLPUBLIC_RTTI StyleSheetUser
 {
 public:
     /** Test whether this object is used.