fix warnings

Author: ludviggunne

test/testsuppressions.cpp

@@ -1785,7 +1785,7 @@ class TestSuppressions : public TestFixture {
                               int thisline,
                               const std::string &xml,
                               const std::string &code,
-                              const std::string expected)
+                              const std::string &expected)
     {
         const char *xmlpath = "testsupressions.xml";
         const char *sourcepath = "test.c";