seccomp: add semctl() to the syscall whitelist

QEMU needs to call semctl() for correct operation.  This particular
problem was identified on shutdown with the following commandline:

 # qemu -sandbox on -monitor stdio \
   -device intel-hda -device hda-duplex -vnc :0

Signed-off-by: Paul Moore <pmoore@redhat.com>
Signed-off-by: Eduardo Otubo <eduardo.otubo@profitbricks.com>