extdom: internal functions should be static
    
    Fixes following compilation warnings:
    ```
    ipa_extdom_common.c:109:5: warning: no previous prototype for ‘__nss_to_err’ [-Wmissing-prototypes]
      109 | int __nss_to_err(enum nss_status errcode)
          |     ^~~~~~~~~~~~
    ipa_extdom_common.c:738:5: warning: no previous prototype for ‘pack_ber_name_list’ [-Wmissing-prototypes]
      738 | int pack_ber_name_list(struct extdom_req *req, char **fq_name_list,
          |     ^~~~~~~~~~~~~~~~~~
    ```
    
    Reviewed-By: Alexander Bokovoy <abbra@users.noreply.github.com>