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

component/proftpd/buildout.cfg

@@ -29,6 +29,7 @@ configure-options =
   --enable-nls
   --enable-ctrls
   --enable-dso
+  --disable-cap
   --with-modules=mod_sftp:mod_ban
   --prefix=${buildout:parts-directory}/${:_buildout_section_name_}
 environment =