common.c: Use _LINUX_CAPABILITY_VERSION_3 in cap_header

While check_capability() function already requires
_LINUX_CAPABILITY_VERSION_3 via "#ifdef" since commit 448627953c we still
set the cap_header's version to deprecated _LINUX_CAPABILITY_VERSION. This
results in a warning like

 > capability: warning: `collectd' uses 32-bit capabilities (legacy support in use)

from the kernel when a plugin (like iptables) calls our check_capability()
function.

With this commit we will set cap_header to kernel's current capability
version (_LINUX_CAPABILITY_VERSION_3), which is default since
kernel 2.6.26.

Signed-off-by: Florian Forster <octo@collectd.org>