pm:cpupower: Add missing powercap_set_enabled() stub function

[ Upstream commit 4b80294f ]

There was a symbol listed in the powercap.h file that was not implemented.
Implement it with a stub return of 0.

Programs like SWIG require that functions that are defined in the
headers be implemented.

Fixes: c2294c14 ("cpupower: Introduce powercap intel-rapl library and powercap-info command")
Suggested-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: John B. Wyatt IV <jwyatt@redhat.com>
Signed-off-by: John B. Wyatt IV <sageofredondo@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>