lib/power: fix make build error

In a fix backported from 21.02, there was no modification to
the Makefile included, as that file does not exist any more.
There is a change needed to Makefile in 19.11 as that still
uses make.

Fixes: b00e2adc ("power: export guest channel header file")

Signed-off-by: David Hunt <david.hunt@intel.com>

lib/librte_power/Makefile

@@ -19,6 +19,6 @@ SRCS-$(CONFIG_RTE_LIBRTE_POWER) += rte_power_empty_poll.c
 SRCS-$(CONFIG_RTE_LIBRTE_POWER) += power_pstate_cpufreq.c
 
 # install this header file
-SYMLINK-$(CONFIG_RTE_LIBRTE_POWER)-include := rte_power.h  rte_power_empty_poll.h
+SYMLINK-$(CONFIG_RTE_LIBRTE_POWER)-include := rte_power.h  rte_power_empty_poll.h rte_power_guest_channel.h
 
 include $(RTE_SDK)/mk/rte.lib.mk