syslog-ng-ctl/Makefile.am: Include more files in make dist

make dist requires control-client.h and control-client-unix.c to be
included as well, add them to the appropriate variables.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #24 from balabit/merge/h/distcheck-control-client

syslog-ng-ctl/Makefile.am: Include more files in make dist