util: gettid() is Linux-specific

util: gettid() is Linux-specific

The manual page clearly states that

  gettid() is Linux-specific and should not be used in programs
  that are intended to be portable.

Unfortunately, it looks like macOS implemented the functionality
and defined SYS_gettid accordingly, only to deprecate syscall()
altogether with 10.12 (Sierra), released last late year.

To avoid compilation errors, call gettid() on Linux only.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>