fix building of libscap on archs w/o live capture support (*BSD, Hurd)

setting eventmask is only supported when we do a live capture, so use the old `HAS_CAPTURE` define to surround them.

Without that patch, the build fails on FreeBSD and Hurd:

* https://buildd.debian.org/status/fetch.php?pkg=sysdig&arch=kfreebsd-amd64&ver=0.1.85-2&stamp=1405266480
* https://buildd.debian.org/status/fetch.php?pkg=sysdig&arch=kfreebsd-i386&ver=0.1.85-2&stamp=1405258294
* https://buildd.debian.org/status/fetch.php?pkg=sysdig&arch=hurd-i386&ver=0.1.85-2&stamp=1405284815

also, Travis really needs a BSD or Hurd build machine :)