nginx: Depends on MMU

This package needs MMU. Hence added dependency on BR2_USE_MMU

Fixes:
http://autobuild.buildroot.net/results/224/2240f11fc52e07ff64913844078e2616842a63dc//



[Peter: also add dependency to menuconfig option]
Signed-off-by: Yuvaraj Patil <yuvaraj.patil@wipro.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>