Make the main() functions in the probing code proper prototypes so that
compiling the probing code with more strict flags won't generate false
negatives.

  <stdin>:1:5: error: function declaration isn’t a prototype [-Werror=strict-prototypes]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221213001653.3852042-8-seanjc@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>