examples/vm_power: fix build with -fno-common

examples/vm_power: fix build with -fno-common

The variables of the same name are merged together
if compiled with -fcommon. It used to be the default.
This default behaviour allows to declare a variable in a header file and
share the variable in every .o binaries thanks to merge at link-time.

If compiling with -fno-common (default in GCC 10), the variable must be
shared as extern to avoid multiple re-definitions.

Fixes: dff22404aaad ("examples/vm_power_mgr: add VCPU to PCPU mapping")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>