virHostCPUGetInfo: Fix build on non-Unix like systems

virHostCPUGetInfo: Fix build on non-Unix like systems

This function is plenty of ifdefs providing implementations for
Linux, *BSD and OS-X. However, if we are being build for any
other architecture, all that's left behind by preprocessor is
just a error reporting call and return of -1. In that case,
passed arguments are unused:

../../src/util/virhostcpu.c: In function 'virHostCPUGetInfo':
../../src/util/virhostcpu.c:966:33: error: unused parameter 'cpus' [-Werror=unused-parameter]
                   unsigned int *cpus,
                                 ^~~~

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>