Fix FILES parameter for "add_samples"

[jloehr]

Description

Fixes the FILES parameter for additional source files in add_samples. There's no SRC_FILES in the current context, the input parameter is called TARGET_FILES.

General Checklist:

Please ensure the following points are checked:

• [x] My code follows the coding style
• [x] I have reviewed file licenses
• [ ] I have commented any added functions (in line with Doxygen)
• [ ] I have commented any code that could be hard to understand
• [x] My changes do not add any new compiler warnings
• [x] My changes do not add any new validation layer errors or warnings
• [ ] I have used existing framework/helper functions where possible
• [x] My changes do not add any regressions
• [ ] I have tested every sample to ensure everything runs correctly

• [ ] This PR describes the scope and expected impact of the changes I am making

  Note: The Samples CI runs a number of checks including:

  • [ ] I have updated the header Copyright to reflect the current year (CI build will fail if Copyright is out of date)
  • [ ] My changes build on Windows, Linux, macOS and Android. Otherwise I have documented any exceptions

  If this PR contains framework changes:

  • [ ] I did a full batch run using the batch command line argument to make sure all samples still work properly