component/proftpd: disable libcap

libcap is a system library that some system may have and some may not,
but we don't want slapos software to depend on the base system.

See also d0a88346
8 jobs for <span class="ref-name">fix/proftpd-missing-rpath</span> in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

04:17:23

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

04:46:02

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

04:26:40

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

05:12:45

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

08:12:33

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

05:31:00

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

04:12:33

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

00:00:40