examples/power: fix FreeBSD meson lib dependency

examples/power: fix FreeBSD meson lib dependency

Samples apps that depend on the power library should not build if
the power library is not present. So now the following apps will
check to see if it's built, else skip building.
* l3fwd-pwer
* vm_power_manager
* guest_cli

Fixes: e0130788576f ("examples/power: support meson/ninja build")
Fixes: 89f0711f9ddf ("examples: build some samples with meson")
Cc: stable@dpdk.org
Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>