libsigrok: enable building C++ bindings

[Thomas:
 - Use BR2_INSTALL_LIBSTDCPP instead of BR2_TOOLCHAIN_BUILDROOT_CXX,
   so that the condition also works with external toolchains.
 - Use a 'if BR2_PACKAGE_LIBSIGROK' instead of duplicating the
   dependency.]

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>