package/powertop: depends on c++

Fixes configure error due to missing c++

checking for library containing pthread_create... no
configure: error: libpthread is required but was not found

Quote from config.log:

configure:21552: checking for library containing pthread_create
configure:21583: false -o conftest    -Os    conftest.cpp -lintl  >&5
[...]
ac_cv_env_CXX_set=set
ac_cv_env_CXX_value=false

using this defconfig

BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_PACKAGE_POWERTOP=y

Please note the number of .cpp files belonging to powertop

output/build/powertop-2.7$ find -iname *.cpp | wc -l
59

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>