libtrading-config: Remove duplicate help text

Reported-by: Marat Stanichenko <mstanichenko@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
libtrading-config: Add zlib to '--libs'

The library depends on zlib so add it to '--libs' of libtrading-config.

Signed-off-by: Pekka Enberg <penberg@kernel.org>