examples/vm_power_manager: fix build

examples/vm_power_manager: fix build

Build fails because '__rte_unused' macro not defined in file, error
produced by 'i686-native-linux-gcc config' but it seems generic issue.

Build error:
.../examples/vm_power_manager/oob_monitor_nop.c:11:13:
   error: expected ‘;’ before ‘static’
   11 | __rte_unused static float
      |             ^~~~~~~
      |             ;
.../examples/vm_power_manager/oob_monitor_nop.c:12:14:
   error: unknown type name ‘__rte_unused’
   12 | apply_policy(__rte_unused int core)
      |              ^~~~~~~~~~~~
.../examples/vm_power_manager/oob_monitor_nop.c:18:21:
   error: unknown type name ‘__rte_unused’
   18 | add_core_to_monitor(__rte_unused int core)
      |                     ^~~~~~~~~~~~
.../examples/vm_power_manager/oob_monitor_nop.c:24:26:
   error: unknown type name ‘__rte_unused’
   24 | remove_core_from_monitor(__rte_unused int core)
      |                          ^~~~~~~~~~~~

Including 'rte_common.h' header which defines the macro for fix.

Fixes: f2fc83b40f06 ("replace unused attributes")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>