zyre: propagate missing BR2_USE_MMU dependency

zyre selects filemq, and filemq 'depends on BR2_USE_MMU', but this
dependency was not propagated to zyre. This commit fixes that.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>