net: ppp: select RING_BUFFER via Kconfig

PPP implementation uses ring_buffer API, but RING_BUFFER Kconfig option
was not selected so far. This resulted in linker errors about undefined
reference to `ring_buf_get_claim' and 'ring_buf_put'.

Add missing RING_BUFFER selection via Kconfig, so PPP driver is properly
built.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>