package/nginx: fix build failure

Correctly set NGX_SYS_NERR, even when it is 0.

Fixes:
  http://autobuild.buildroot.org/results/666/666d780bf61e10c9f89f765bb04ddc1afc11d0d1/
  http://autobuild.buildroot.org/?reason=nginx-1.6.2&&arch=powerpc



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>