package/nginx: remove rtsig module support

package/nginx: remove rtsig module support

rtsig module uses the deprecated sysctl syscall, which is not
implemented on some recent architecture (such as arc).

Also, rtsig module is almost considered as deprecated by some nginx
developers [1].

So, just remove this option.

Fixes:
  http://autobuild.buildroot.org/results/730/730105fc0a89b381b3b29192d07f28ef1f13cbb3/

[1] http://mailman.nginx.org/pipermail/nginx-devel/2014-November/006295.html

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