coresight: Fix Kbuild options for CORESIGHT_OST

Enabling Coresight OST feature on its own might lead to
compilation issue. This patch makes sure that Coresight OST
feature can only be enabled when Coresight STM is defined.

Change-Id: I0a5e694da7d110ee2a122c62f96a24a64f486c9d
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>