SymbolResolver: Use filename when path cannot be found

Uses bare filename to pass to `dlopen` when the full path cannot
be detected. This helps on systems where library paths are not the
same as $PATH.

src/Runtime/SymbolResolver.cs

@@ -80,8 +80,8 @@ namespace CppSharp
                         break;
                     }
                 }
-                if (!File.Exists(attempted))
-                    continue;
+                if (attempted == null)
+                    attempted = filename;
 
                 var ptr = loadImage(attempted);