[FS#254] Build error with additional libustream library (check_data_file_clashes)

[LEDE Bugs]

The following task is now closed:

FS#254 - Build error with additional libustream library (check_data_file_clashes)
User who did this - Jo-Philipp Wich (jow-)

Reason for closing: Not a bug
Additional comments about closing: Multiple libustream instances with different SSL backends cannot coexist.

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=254