Fixed build on Linux with x32 ABI.

On Linux x32 inclusion of sys/sysctl.h produces an error.  As sysctl() is
only used by rtsig event method code, which is legacy and not compiled
in by default on modern linuxes, the sys/sysctl.h file now only included
if rtsig support is enabled.

Based on patch by Serguei I. Ivantsov.