Tests: Fix Plugin test on Watcom compiler

The Watcom compiler does not have stream operators for std::string.
Since KWSys no longer provides the operators for us, just use c_str()
to avoid the problem and allow the test to compile on Watcom.

Tests/Plugin/src/example_exe.cxx

@@ -32,7 +32,7 @@ int main()
   if(!handle)
     {
     std::cerr << "Could not open plugin \""
-              << libName << "\"!" << std::endl;
+              << libName.c_str() << "\"!" << std::endl;
     return 1;
     }
   kwsys::DynamicLoader::SymbolPointer sym =