zeromq: Remove useless space

Be coherent with all the other ifeq check where there's no space.
>From the doc of Make, it should not cause any trouble, but coherence has
its own merit.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>