subsys/bluetooth/host/keys.h: Add include guard and required include

This header requires bluetooth.h but was not including it itself.
Due to this we had 2 tests failing to build.
Let's just include the dependencies to this header instead of
relaying on users including the dependencies dependencies in
the right order.

Also, let's add an include guard as in a test this header was
included twice leading to weird build errors.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>