Add CAP_SETPCAP to CapabilityBoundingSet in irqbalance.service

A error message of 'libcap-ng used by "/usr/sbin/irqbalance" failed dropping
bounding set due to not having CAP_SETPCAP in capng_apply' is noticed.

Previously a similar issue[1] has been fixed by the following commit:

    efab272 Drop CapabilityBoundingSet from irqbalance service
    43751df drop NoNewPrivs from irqbalance service

in which, CapabilityBoundingSet and NoNewPrivs parameters are dropped
from the irqbalance.service, and get restored by the following commit
later:

    a99b604 Set additional systemd options for service

So this patch will not do the dropping again, but add CAP_SETPCAP to
CapabilityBoundingSet instead.

[1]: #182

Signed-off-by: Tao Liu <ltao@redhat.com>

misc/irqbalance.service

@@ -9,7 +9,7 @@ ConditionCPUs=>1
 EnvironmentFile=-/usr/lib/irqbalance/defaults.env
 EnvironmentFile=-/path/to/irqbalance.env
 ExecStart=/usr/sbin/irqbalance $IRQBALANCE_ARGS
-CapabilityBoundingSet=
+CapabilityBoundingSet=CAP_SETPCAP
 NoNewPrivileges=yes
 ProtectSystem=strict
 ReadOnlyPaths=/