Make suricata_ctl_flags be volatile

The global variable suricata_ctl_flags needs to volatile, otherwise the
compiler might not force the variable to be loaded every time it is used. This is required because the
variable can be written by other threads.

This was causing Suricata to not exit under some conditions.

- PR build: https://buildbot.openinfosecfoundation.org/builders/ken-tilera/builds/32
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/ken-tilera-pcap/builds/38