fix --disable-poppler build with clang plugins enabled

Change-Id: Ibc133de32a8c4343c5d941f78933cb8bae24f434
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/178937
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>

sdext/qa/unit/pdfimport.cxx

@@ -60,6 +60,7 @@ using namespace ::com::sun::star;
 namespace
 {
 
+#if HAVE_FEATURE_POPPLER
     class TestSink : public ContentSink
     {
     public:
@@ -462,6 +463,7 @@ namespace
         bool                      m_bDashedLineSeen;
         bool                      m_bImageSeen;
     };
+#endif
 
     class PDFITest : public test::BootstrapFixture, public XmlTestTools
     {