nodeinfo: fix build on non-Linux

nodeinfo: fix build on non-Linux

Commit b0f8546 broke the build on mingw, by exposing code that
had Linux-specific dependencies but which was previously protected
by libnuma ifdef guards:

make[3]: Entering directory `/home/eblake/libvirt-tmp/build/src'
  CC       libvirt_driver_la-nodeinfo.lo
../../src/nodeinfo.c: In function 'virNodeGetSiblingsList':
../../src/nodeinfo.c:1543:30: error: 'SYSFS_THREAD_SIBLINGS_LIST_LENGTH_MAX' undeclared (first use in this function)
     if (virFileReadAll(path, SYSFS_THREAD_SIBLINGS_LIST_LENGTH_MAX, &buf) < 0)
                              ^
../../src/nodeinfo.c:1543:30: note: each undeclared identifier is reported only once for each function it appears in
../../src/nodeinfo.c: In function 'virNodeCapsFillCPUInfo':
../../src/nodeinfo.c:1562:5: error: implicit declaration of function 'virNodeGetCpuValue' [-Werror=implicit-function-declaration]
     if ((tmp = virNodeGetCpuValue(SYSFS_CPU_PATH, cpu_id,
     ^
../../src/nodeinfo.c:1562:5: error: nested extern declaration of 'virNodeGetCpuValue' [-Werror=nested-externs]
../../src/nodeinfo.c:1562:35: error: 'SYSFS_CPU_PATH' undeclared (first use in this function)
     if ((tmp = virNodeGetCpuValue(SYSFS_CPU_PATH, cpu_id,
                                   ^
cc1: all warnings being treated as errors

* src/nodeinfo.c (virNodeCapsFillCPUInfo): Make conditional.
(virNodeGetSiblingsList): Move into #ifdef linux block.

Signed-off-by: Eric Blake <eblake@redhat.com>