powerpc32 compilation failures due to char signedness issues.

Originally reported in: https://lists.torproject.org/pipermail/tor-dev/2015-February/008231.html

One of the scheduler functions uses a char as an argument to pass a -1/1 value, which causes GCC to issue warnings on platforms where char defaults to unsigned. While the patch does address the issue, after discussion with nickm, the argument is better changed to an int.