[2726] Pass plain old C string

This should prevent cppcheck from complaining on several places about
passing c_str() as string argument. It is not possible to remove the
c_str() from the macro, as it might be needed at other places. Putting
the suppression at several places seems wrong, so this is the least ugly
solution.

src/lib/config/config_data.h

@@ -29,7 +29,7 @@ namespace config {
 /// point to anything defined in the .spec file)
 class DataNotFoundError : public isc::Exception {
 public:
-    DataNotFoundError(const char* file, size_t line, const std::string& what) :
+    DataNotFoundError(const char* file, size_t line, const char* what) :
         isc::Exception(file, line, what) {}
 };